Luke: Hell no. They outlawed that in this state two years ago.
It isn't at all unusual for siblings to be close to each other. Lots of siblings will hang out with each other and show some sibling affection towards each other. If someone else sees them and knows that they're related, they won't raise any eyebrows.
If they do not know that they're related, they might think the two are dating.
This trope is often used as a type of The Reveal where the audience does not know that the two siblings are related beforehand, and are led to believe that the person is cheating or likes someone else.
And, of course, this is most definitely Truth in Television.
This is somewhat the inverse of, but definitely comparable to, Like Brother and Sister—in which they have a close, platonic, but non-genetic relationship. If the characters themselves don't know, it's Surprise Incest. If their resemblance to a romantic couple is particularly convincing, there may be Incest Subtext at work; if it's more convincing than the idea that they're brother and sister even to the audience who's already in on the secret, that's probably Platonic Writing, Romantic Reading. Sometimes though, it happens to be Exactly What It Looks Like.
This can happen with cousins, as well. With parents, it may overlap with Absurdly Youthful Mother: if a character's parent doesn't look old enough to be their parent, mistaking them for a romantic partner is all the more likely.
Warning: Spoilers Ahead.
- In one of the many commercials that John Stamos (Uncle Jesse from Full House) did for Dannon Oikos yogurt, the woman he was seeing had apparently broken up with him and was surprised to see how well he was taking it as he indulged in his yogurt. But when an attractive woman then walks in, she is suddenly aghast that he seemingly found another girlfriend so quickly, smashing a picture. He informs her that the other woman is actually his sister and that the picture she smashed happened to be of their mother.
- Implied in Attack on Titan: After the death of his prostitute mother, Levi is taken in by Kenny Ackerman, who simply states that he knew her in the past. As time goes on Levi finds evidence that he is also part of the Ackerman family, and while not stated explicitly, it's easy to read Levi as suspecting that Kenny might be his father. When Levi finally asks Kenny how he knew his mother, he laughs and explains that she was his sister, making him Levi's uncle (something which the audience already knew).
- In Baka and Test: Summon the Beasts, Akihisa attempts to keep his sister who has just recently moved home hidden from his friends for fear of her embarrassing him. When they start getting suspicious and go to his house they believe that he has a secret live-in girlfriend, much to Himeji's denial: "Akihisa-kun, this bra is way too big for you. You need a smaller cup size."
- Betrayal Knows My Name: Toko and Tsukumo Murasame seem to fit this, even going so far as to sleep in the same bed.
- Cardfight!! Vanguard: In Episode 5, when Emi yells at Aichi for not doing his homework with her, Kamui thinks thinks the two are a couple. They're actually siblings.
- Demon Slayer: Kimetsu no Yaiba: Zenitsu mistakes Nezuko for Tanjiro's lover instead of sister and attacks Tanjiro out of jealousy before he learns the truth.
- Used in Dragon Ball Z: Krillin mistakenly believes that Android 17 and 18 are a couple rather than Half-Identical Twins.
- A horrific one occurs in Elfen Lied, where Kaede mistakes Kouta's sister for his girlfriend. She promptly kills her and his father.
- Inverted in Even Though We're Adults. Akari initially mistakes Ayano's husband for her brother. She didn't know that Ayano was married.
- Invoked with Miki and her brother Kazuki in Fresh Pretty Cure!. Love, Inori, and even Miki's admirer Yuuki mistake Kazuki for her boyfriend on multiple occasions, and he plays along to keep other boys from hitting on her.
- In Haikyuu!!, Tanaka sees Kanoka with an older man and assumes he's her boyfriend, much to his embarrassment (he was in the midst of rejecting what he thought was a love confession from Kanoka). It's only after he runs away that the audience finds out the man is her cousin. Tanaka himself finds out the next day.
- In Himouto! Umaru-chan, when Umaru and Taihei go out in public, bystanders often get jealous of Taihei, thinking that the beautiful Umaru is his girlfriend instead of his sister.
- Shiba Tatsuya and Shiba Miyuki from The Irregular at Magic High School are a pair of rather affectionate siblings. So much so that even their innocuous conversation with each other creates an atmosphere that makes others think about incest. It is no wonder that Ichijou Masaki mistook them for lovers when he first met them. His startled realization that they are actually siblings is Played for Laughs, at the same time, in the future, we first learn about the incestuous feelings of Miyuki, and then that Tatsuya is ready to try to love her as a woman. In general, at the moment they are engaged after a series of events ...
- Kenichi: The Mightiest Disciple: When Renka shows up, she furiously attacks Kensei. Given that Kensei is a Dirty Old Man, Kenichi immediately assumes that Renka is a disgrunted ex-girlfriend. An annoyed and disgusted Kensei explains that Renka is his daughter who is attacking him for abandoning the family.
- Zig-zagged in K-On! High School when the other Pop Music Club members see Jun meet a guy. Three of them think he's her boyfriend, but Ui is unsure, having heard she has a brother, but they assume "boyfriend status confirmed" when she squeezes his arm. But then they learn after confronting them that he actually is her brother Atsushi.
- In Love Monster, Hiyo assumes that Kurou is flirting with Chiya, but in reality, he just found out Chiya is his sister. Later on in the story, a squicky situation occurs where Chiya does actually try to pursue a relationship with Kurou, which is perfectly acceptable in monster society. Kurou is not interested, however.
- Macross 7 has a variation when Mylene visits Max Jenius on the bridge to talk to him. She's never been on the bridge before, and after they leave to talk in private, the Bridge Bunnies go into a frenzy over Max's new "girlfriend" who is so much younger than he is, some of them scandalized and others grudgingly admiring Max for attracting someone so young and beautiful. Then Exsedol cuts in and informs them all that Mylene is Max's daughter. (Mylene's somewhat risque manner of dress probably wasn't helping matters.)
- In My Love Story!!, a man named Oda confesses to Ai and gets turned down because Ai is still in love with Takeo. He goes to meet Takeo however mistakes Suna, Ai's Bishōnen younger brother, for Takeo. Ai described Takeo as very attractive looking however Takeo isn't what Oda expected.
- It is possible to see this trope at work in Rebuild of Evangelion, counting Rei's affection for Shinji as purely maternal — if this is the case, Asuka makes a fateful error.
- Sailor Moon:
- Used in the English dub to Hide Your Lesbians. Serena and Mina mistake Amara for a man and assume "he" is dating Michelle when they are actually cousins. Of course in the original Japanese they were a couple and the dub didn't edit out that many couple-y scenes between them.
- Another variation in the R season. Serena/Usagi spots Darien/Mamoru giving another girl a lift and assumes he's found another girlfriend. The girl turns out not to be his sister but rather Andrew/Motoki's sister though the set-up remains the same. The sister has a boyfriend already anyway.
- School Rumble plays with this one: After Harima sees Tenma and Karasuma together, he sulks in his room and ends up watching "Comet Wars", which mirrors the Star Wars use of this trope. Harima suddenly realizes he may have made this mistake and that Karasuma is Tenma's brother... but later realizes he was just fooling himself.
- Invoked in A Silent Voice: Shouya meets a boy who says he's Shouko's boyfriend. The awkward part is that Shouko is around seventeen while the kid is a middle schooler. As in turns out the two are related, with Yuzuru being Shouko's overprotective little sister.
- In one episode of Tokyo Magnitude 8.0, a man mistakes Mari for Yuuki's and Mirai's mother. She doesn't react negatively and when Mirai complains about it later she just mentions her own young daughter.
- Variation in Tonari no Kashiwagi-san. Yuuta sees his crush Kotone with Kotori's husband Takuya and initially believes that there's cheating afoot, unaware of the fact that Kotone and Kotori are sisters. The misunderstanding gets cleared up almost immediately.
- True Tears, Episode 3: Shinichiro sees Noe ride off on a motorcycle with another guy, who he assumes is her boyfriend. He finds out otherwise when he takes her to a basketball game and sees the same guy...
Shinichiro: [to his guy friend] That's him! That number four. He's Noe's...
Shinichiro: ...brother. [pause] Brother?!
- In Weiß Kreuz, Omi and Sakura both assume that Aya-chan is their Aya's lover when they find out he's been taking care of a girl in a coma and using her name. The Assassin and White Shaman manga has a scene where Aya-chan forces her brother to leave his job early and buy her something to eat, and his coworker, implying that this has happened before, assumes she's his girlfriend.
- One Archie comic has Archie and Reggie seeing Veronica hanging around with a hot guy. They jealously assume he's competition and spend the story trying to one-up him. In the end, when they're caught, a decidedly unimpressed Veronica informs them that the guy they saw her with was actually her cousin.
- Batman had an example in the Golden Age, with the King of Cats, Catwoman's brother.
- The Sandman: In the Endless Nights story "Destruction - On the Peninsula", Destruction and his sister Delirium wander near an archeology team's dig site. One member of the team assumes they are a couple and derisively says he's too old for her, while another member assumes they are father and daughter. One member, Rachel, talks to both of them and finds out they are siblings.
- In Star Wars: Invasion, Finn Galfridian assumed Jaina Solo and her brother Jacen were a couple.
- Superman and Supergirl are cousins and they are (usually) extremely close as the last Kryptonians and each other's only (living) biological family. They are so close that people unaware of their kinship assume that they are lovers. Some examples:
- In Action Comics #500 Superman explains to several fans how he came to Earth, became a hero, met his cousin... A member of the audience then blurts out: "I never knew Supergirl was your cousin — I guess I always thought she was your girlfriend!"
- In Supergirl vol 2 #20, Linda Danvers (Supergirl's secret identity) tells her friend Joan that she's got a date for lunch with Clark Kent. When Joan starts gushing about her hot date, Linda explains that Clark is her cousin.
Linda: 'Fraid you're on your own today, Joanie. I've got a date for lunch.
Joan: With the gorgeous Mr. Philip Decker, I presume—?
Linda: Nope. A different man... named Clark Kent.
Joan: Clark Kent... THE Clark Kent, the news guy from GBS?! Don't you ever see guys who aren't nationally famous?
Linda: Relax, Joan. Clark's my cousin!
- In Supergirl vol. 6 34 (a Superman: Doomed tie-in), a guy asks Supergirl whether she's Superman's girlfriend.
Michael: I mean, I don't know exactly how you and Superman are connected, but I can tell he matters to you. He's... What? Your brother? Your boyfriend...?
Supergirl: ... My cousin.
- Last Daughter of Krypton: After seeing Kara in action, Symon Tycho remarks her physiology, costume and even chest insignia are similar to Superman's, and he wonders if she is his ex-girlfriend.
Simon Tycho: I'm starting to think the symbol she's wearing isn't a coincidence. Is she related to Superman somehow? Sister? Maybe an ex-girlfriend?
- This, along with an extra serving of Mistaken for Gay, happens to brothers Cyclops and Havok in an issue of X-Factor.
- Luann: In one arc, Brad, smitten with Toni, goes to a car show where she's at but sees her with some more handsome guy. Brad is upset, but it later turns out the guy is her brother — although it also later turns out that Toni does in fact already have a boyfriend.
- In one Blondie strip, Blondie and Dagwood's kids are at the beach. Alexander offers to get Cookie a cold drink, and while he's en route to the refreshment stand, another guy stops him and compliments him on his luck in being with such a beautiful girl. The bemused Alexander explains that she's his sister.
Guy: (immediately changing directions) In that case, I'm the lucky one!
- In Dragon Ball Z Abridged, Bulma ends up straight-up propositioning Trunks, unaware that he's her Kid from the Future. Once she discovers the truth three years later, thanks to Piccolo, it takes a while to realize what she tried to do. When he gets back to his own time, he brings it up with his mother, and she's perfectly understanding of her alternate younger self:
Future Bulma: Well, duh!
Future Trunks: MOM!
Future Bulma: Hey, it's not my fault your dad's genetics and mine got along like chocolate and peanut butter.
Future Trunks: Is that why you used to call me your "little peanut butter cup"?
- In the Spirited Away fanfic Resilience of the Soul, Linca's Backstory involves the time she became upset when she saw her lover hugging a pregnant woman who turned out to actually be his sister. It's not funny though — before the matter can be cleared up, she accidentally kills him.
- Humorously subverted in the Buffy the Vampire Slayer/Supernatural fanfic Runs in the Family, where Xander asks Sam and Dean if they're brothers, which Dean interrupts with "Dude! We are NOT gay!" Given the number of times they've been asked if they're together in the actual series, his reaction is understandable.
- In Hero High, many people think that Ben and Gwen are a couple. Though it doesn't help that the author enjoys adding the Ship Tease.
- In the Buffy the Vampire Slayer/NCIS fanfic Little Brother, Xander Harris answers the door of his hotel room to find Gibbs and Tony waiting for him, wanting to ask if he knows anything about a murder in the same hotel the night before. When they hear their teammate Abby in the same room, sounding as if she's in pain (she had just woken up with a hangover), they flip out and nearly arrest Xander on the spot. It takes a few minutes until Abby herself sees this, and her angry reaction is "You cuffed my little brother?"
- This Avengers High School A.U., where Thor apparently forgot to tell all his friends he has a younger brother. Doesn't help that Loki is clearly not blood-related.
- Six Brides For Two Sisters: Combined with Mistaken for Pedophile when Princess Cadence and Shining Armor recall how they first met: she saw him walking away with the filly she was foalsitting with promises of fun and candy, but since Twilight and Shining don't have any family resemblance (despite being siblings), she jumped to conclusions. Violently.
- In Over The Edge And Through The Wood, Troy assumes Applejack and Big Macintosh are Applebloom's parents instead of her siblings.
- In The Lunar Rebellion, Nimbus Kicker thinks her lover Radiant Day is cheating on her with his squire White Knight. Turns out, White Knight is his sister. And also his squire. But most importantly of all, his sister.
- In Ever After High fanfic Poisoning Apple, Raven and her older brother Rotbart don't look anything alike and no one actually knows that they share the same mother. Apple gets the impression that Rotbart and Raven are romantically involved to explain why Rotbart took Raven's place as Snow White's villain and why Raven is affectionate to him and spends time with him. This is seemingly confirmed when Rotbart gives Raven the very romantic gift of his mother's jewelrynote for her birthday.
- In Avenger Goddess, while Diana isn't a biological relative of Howard Stark or Natasha, she considers them her family to the point that she is disgusted and outraged at the implication that she might have been romantically involved with either. When a teenage Tony accuses her of being Howard's mistress after Howard's funeral, she is outraged and disturbed at the idea that she had that kind of relationship with Howard when she actually saw him as a brother, and when Clint first meets Diana and Natasha, he initially wonders if they're dating and is surprised at the intensity of disgust they express at that idea (Diana is actually Natasha's adopted mother).
- Inverted in the Frozen fanfic Never Been Bothered, where the protagonist mistakes Anna and Elsa for being sisters instead of a couple. Later subverted when it turns out they are siblings. They fled their kingdom, along with Kristoff, Sven, and Olaf, to be together.
- Pony POV Series: The Cutie Mark Crusaders thought Prince Blueblood was Princess Cadence's ex-boyfriend. They are cousins.
- In Returning Echoes, a jealous Winry slaps Alphonse and runs off when he brings home a blond woman, unaware said woman is Edward.
- Fate/Harem Antics: Rin Tohsaka thinks that Caren Hortensia is Kirei Kotomine's lover instead of his daughter. Since Caren hates Kirei, she has no problem playing it up to make him look like a Pedophile Priest.
- A vendor in My Master Ed assumes Edward and Hohenheim are brothers. Hohenheim is under the impression that their similar appearance is a coincidence and they aren't related at all. Only Edward himself knows they're father and son.
- In Juxtapose, Ochako thinks Toshinori is Izuku's step-dad because of how close they are. Hitoshi decides to perpetuate this misunderstanding to laugh at Izuku's flustered reaction. Later on, Izuku admits he sees Toshinori as the father figure he never had.
- When Oscar sees Ruby and Yang hugging in RWBY Thoughts, he mistakes them for girlfriends instead of sisters.
- Of Monsters and Men: Cornelia thinks this when she hears that her sister kissed a strange man. She doesn't realize that the man is their long-thought dead half-brother Lelouch and that the kiss was purely platonic.
- A Song of Grumbling & Cotton Candy: Pizzazz mistakes Blaze's voice for Video's and accuses Clash of dating Video. Video and Clash are cousins.
- In one of doodlegirll's stories, Wilbur Robinson's girlfriend catches him embracing a pretty girl and runs off in tears, assuming he's cheating on her while the readers know said pretty girl is Wilbur's daughter with said girlfriend, back in time to hang out. Of course, Fridge Logic sets in when Wilbur doesn't tell her this since his girlfriend is fully aware that time travel exists.
- In story #15 (Scars of the Quill), when Flash Sentry arrives in town, he's greeted by Twilight, Quiver Quill and Sour Sweet, the latter of whom he went through guard training with. He promptly teases Quiver by saying "Now please tell me you and Sour aren't dating." Quiver quickly informs him that they're cousins.
- In story #17 (Many Happy Reunions), this happens to Quiver and Sour again. Sunrunner, Quiver's old classmate (and who has feelings for him), arrives at his house and finds Sour Sweet, whom she knows from their time in the Guard together, answering the door. Grimly, she asks why Sour is there, and Sour immediately replies that she lives there, then demands to know "why you took so damn long getting back in touch with my cousin". Sunrunner is quite visibly surprised, but also clearly relieved, to hear that they're related rather than dating.
- In Comes and Goes (in Waves), everyone believes that Five and Raithe (Ghost Klaus) are together. Everyone. It doesn't help that they're Cuddle Bugs who have no trouble with casually saying a Platonic Declaration of Love whenever the mood strikes them, thanks to spending over two decades in the apocalypse with only each other as company. They're aware of and extremely Squicked by the misconception (especially since Raithe pretty much raised Five), but it's so pervasive they hardly even try to correct it anymore.
- In Burning Bridges, Building Confidence, Cole is a cousin of Marinette's who moves back to Paris and transfers to her school. Their classmates aren't aware of their relationship at first, and when Adrien tries to convince Marinette to stop trying to expose Lila's Malicious Slander, he refers to Cole as her 'girlfriend'.
- A Diplomatic Visit: In chapter 20, when Twilight refers to "Shiny", Maxilla teases her about it, saying "Getting a little familiar with your fellow princess's husband there, aren't you?". A rather indignant Twilight promptly informs Maxilla that Shining Armor is her older brother.
- Young Justice: Darkness Falls: In chapter 6, Starfire explained the reason why she kissed Nightwing (to acquire his language) instead of Conner was that the latter was with his mate (M'Gann, whose relationship with Conner is... complicated at that point) and their offspring (Beast Boy, who is really M'Gann's adopted brother).
- In the Persona 3 fic Wh-WHAAAAAAAAAAT, a rumor is spread around Gekkoukan High School that Makoto and Yukari have a child after they were spotted hanging around a child by the Naganaki Shrine, who Makoto referred to as their family. The child in question was Ken; as Aigis points out, he is ten years old, while Makoto has been in Iwatodai for less than a year and has only been dating Yukari for three months. In addition, though it isn't stated, Yukari and Makoto would have been several years younger than Ken is now when he was born.
- The divergences in With This Ring include Superboy being adopted by Wonder Woman, who of course looks not much older than him (and people do look). Some of his school peers wonder if he's cheating on "Megan" (Miss Martian) when they see Diana hug him and kiss his forehead, until they find out what's actually going on.
- At the beginning of the the 2005 ''Muppets fic Swamp Call, Kermit's sister Maggie comes to visit from the swamp. Not knowing who she is, Piggy is jealous, and Maggie, being The Prankster, teases her by pretending to really be Kermit's girlfriend, until Kermit arrives and reveals the truth.
- In Miraculous Ladybug fanfic Blessed, the second chapter features an example that majorly backfires on the one who makes it. Compulsive liar Lila Rossi, in an attempt to make Marinette and Nathaniel look bad (since they're the few who are willing to stand up to her and call her out on her lies), tells some of her classmates that she caught Nathaniel threatening Marinette to stay with him romantically. This is what finally makes the other students recognize her for the liar she is — they all know, though Lila didn't, that Marinette and Nathaniel are fraternal twins.
- In The Awakening of a Magus, Sirius gets angry when he sees Adelaide talking to Severus, only for the latter to point out that first, they are cousins (albeit distant ones), second, they were also good friends, and third, that who is Sirius to talk after breaking Adelaide's heart while at school.
- In the Fate/Grand Order fan comic How Complicated Can This Family Tree Get, Artoria finds Lostbelt Morgan and Lancelot and accuses him of making a move on her sister. Lancelot protests that, one, he wasn't making a move on anyone, and two, that he was speaking to his adoptive mother Nimue, much to his king's confusion. Morgan then says that the Proper Human History versions of Morgan and Nimue are both the same person, specifically split personalities. Lancelot then leaves after realizing that the king he secretly held impure thoughts towards (including getting her older self into a bunny suit) was actually his adoptive aunt.
- One One Piece fan comic has Boa Hancock watching Luffy being affectionate with Anne (A Gender Flip Ace) with jealousy, even thinking that Luffy might prefer Anne's petit chest over Hancock's more volumptious pair. She doesn't know that Anne is Luffy's adopted older sister.
- In the Punch-Out!! fanfic Ma Fille chapter "Sweet Baby Sakura", Natsumi's doula, Nurse Mahari, mistakenly believes Piston Hondo to be her partner. Hondo, in a rare moment of immaturity, is grossed out and clarifies he's her brother. Mahari trolls him a bit by pointing out the age gap between him and Sakura, acting like she thinks Hondo is Sakura's brother and not Natsumi's.
- At one point in Fate/Noble Shade, the protagonist sees Rin and Sakura interacting and assumes they're secretly girlfriends. Much later, he becomes privy to the fact that they're secretly sisters.
- In Miraculous: Tales of Scarlet Beetle & Ikati Black, Kim was disturbed finding out that the seemingly older sculptor Théo Barbot had a crush on his superhero identity Scarlet Beetle. So when he sees Théo at his school and hugging his schoolmate Mireille, he freaks out and is about storm over to protect her until he sees Mireille hug him back in a familiar way. Kim then learns from Ivan that Mireille and Théo are half-siblings, and that he's from an art collège (a middle school) a few blocks from their own school.
- Inverted in After The Dark where it's mentioned that Jack once brought his boyfriend to a party and people thought they were cousins.
- In Archie: To Riverdale and Back Again, Jughead's son gets mistaken for Archie's son by his teachers at their high school reunion.
- In Bugsy, Bugsy Siegel threatens to beat up a man with his pants off in his girlfriend's house. She's mending a hole in her brother's pants pocket.
- In The Dilemma, Ronny mistakes a man talking to his girlfriend as trying to feel her up. But when he confronts them, Ronny's girlfriend says that the man is actually her cousin — and he's also gay.
- Doom: The team of marines has a John Grimm. They meet a Hot Scientist named Sam Grimm and she and John clearly know each other. John's teammates assume she is his ex-wife until he explains she is his sister.
- In the 2009 film Dorian Gray, when Dorian introduces himself to Sybil Vane for the first time, he says he saw her previously in a gin tavern but was unable to talk to her because she'd left with another man. He's noticeably uncomfortable, assuming the man was her lover until she says that the man she left with was "Jim... my brother."
- G Men: When Brick, who has become attracted to Kay, sees her kissing Jeff (his boss), he's unhappy at first, but when he learns Kay is Jeff's sister, he smiles.
- The plot catalyst in Gray Matters. The brother-and-sister team both kiss the same woman, too.
- Kull the Conqueror: Kull assumes Ascalante to be Zareta's lover since he's so keen on rescuing her. When he asks Zareta directly about their relation to each other, she reveals that Ascalante is her brother. She later becomes Kull's lover, and Ascalante is totally fine with that.
- In Letters to Juliet, this causes Sophie some grief when Charlie introduces her to a woman named Patricia, who Sophie initially assumes is the ex-girlfriend he had mentioned earlier in the film. It turns out that the Patricia who Charlie was introducing Sophie to was actually his cousin, who just happens to have the same first name as the ex-girlfriend he had previously mentioned.
- Inverted in the Winona Ryder film adaptation of Little Women. A suitor of Jo's calls at the house, where Hannah the housekeeper mistakes him for a European friend of Amy's and informs him that "Miss March and Mr. Laurie are livin' next door". Friedrich takes this to mean that Jo has married her old friend Laurie, but Jo hurries down the road after him to explain that it's her sister who is married.
- Mabel's Blunder: Mabel sees her boyfriend embracing a woman and gets the wrong idea. It was his sister, and it was an innocent hug. But slapstick hijinx ensue nonetheless.
- In Mystic Pizza, Daisy is outside a country club with Kat and Jojo when she sees Charlie, her boyfriend, inside with a beautiful woman. In a rage, Daisy pours two barrels of fish into his car. Then Charlie introduces the woman (named Serena) as his sister.
Daisy: [crying] I fucked up.
Charlie: Yeah, but you gave it 100% effort.
- In Parasite, the Parks' daughter Da-hye notices that her English tutor Ki-woo is more familiar with her little brother's art teacher Ki-jeong than he should be. He'd told the Parks that she was a friend of his cousin's. She thinks they're dating but they're actually siblings.
- The French movie Qu'est-ce qu'on a fait au Bon Dieu uses this early on. An upper-class, white Catholic family has four daughters, three of whom have married an Algerian Muslim, a Sephardic Jew, and a Chinese Taoist. The final daughter is in a relationship with a Catholic... who's black. The three brothers-in-law take pictures of the fiancé with an attractive woman outside a hotel while in Paris, blissfully unaware of the hypocrisy of assigning stereotypes. When they show the sister the pictures, she asks what the hell is wrong with them, that's his sister. They genuinely apologize to him and do what they can to reconcile the families (as the fiancé's father is as opposed to his son marrying a white girl as her father to his daughter marrying a black guy).
- Slaughterhouse Rulez: Clemsie spends a lot of time hanging around Smudgers, touches his arm, and mentions that she loves him. The final act reveals that the two are siblings, not lovers.
- Star Wars VI: Return of the Jedi: Han briefly fears that Leia's avowed love for Luke puts him out of the picture, and promises not to "get in the way." But having discovered Luke is her brother, Leia is able to reassure him. (Of course, considering how vigorously she kissed Luke in the preceding film, Han's doubts weren't exactly unreasonable.)
- Tango & Cash: Cash initially assumes Tango's sister to be his girlfriend. For that matter, so did probably most viewers, since it's never spelled out until Tango accidentally walks in on them. Which, when you think about it, unwittingly makes Cash far more of an asshole when he starts flirting with her behind Tango's back than he would otherwise be.
- Used in The Thomas Crown Affair, where the young hot girl he's seen dancing with and was in his bedroom is actually his adopted daughter. He could've easily told Catherine this, but he triggered this trope because he wanted to test her.
- A major plot point in Thoroughly Modern Millie. Millie sees Jimmy sneaking off at night with Miss Dorothy and assumes the worst. The movie ends with her realizing that she loves Jimmy...only to see him going off at night with Miss Dorothy and Muzzy. When she coolly confronts him and asks if she can join them, Jimmy cheerfully invites her to join him, his sister, and his stepmother. It turns out the three pretended they were unrelated because Muzzy wanted her stepchildren to find people who loved them for themselves.
- Early on in With Six You Get Eggroll, Abby McClure and Jake Iverson, both widowed with children, make a date for Friday night. Jake calls to cancel, citing his daughter's birthday, so Abby goes to a disco with her sister and brother-in-law, where they see Jake dancing with a pretty young girl. Abby gives Jake the cold shoulder but later learns that the pretty young girl is actually Jake's daughter when she sees her at her son's high school graduation.
- In Zathura, when Lisa finds out the astronaut she is attracted to throughout the movie is actually her little brother several years later, she cries, "Oh, my God! And I wanted to--" before an explosion cuts her sentence off. Needless to say, she was squicked and befuddled by it all.
- Sue Townsend's Adrian Mole, when aged fifteen, is allowed to witness the birth of his sister because the nurses assume he's actually the baby's father.
- In Bag of Bones by Stephen King, Mike learns that before his wife Jo's death, she had been seen hanging around TR-90 with a man and being openly affectionate with him. He naturally assumes that his wife has been having an affair until he confesses his fears to Jo's brother Frank, who reveals that he was the man with whom Jo had been seen.
- The Berenstain Bears: The chapter book The Berenstain Bears and the School Scandal Sheet has classmate Queenie McBear put an unapproved article in a secondary (and unauthorized) school newspaper about Teacher Bob being seen having dinner with a pretty teenager, causing a fuss and an inquiry for unprofessional behavior. Queenie's editor-in-chief Brother Bear angrily informs her that had she not gone over his head (which ultimately led to them being exposed as the writers of the secondary newspaper), he would have killed the story on the spot — he knew full well that the girl in Queenie's photo was Bob's teenage niece, who was in town to visit him.
- In David Eddings' The Belgariad, Ce'Nedra makes this mistake about Belgarion and his cousin. Her response to being corrected is, amusingly, equal parts "Whew, he's not attached" and "Shit, I'm in love with him!"
- In one of Kathy Reichs' Temperance Brennan novels, several other characters mention seeing her boyfriend Ryan going out with a young woman. It's eventually revealed that she is his recently discovered daughter.
- In The Count of Monte Cristo, Debray assumes Albert and Mercedes are having an affair after running into them at a seedy inn. The fact that Mercedes is veiled partly excuses the fact that he mistook his friend's mother for his mistress. There's also an earlier scene where the Count sees a painting of a younger Mercedes in peasant dress and makes a comment about her being Albert's mistress. While he's only pretending to make that mistake and actually knows who Mercedes is, it's kind of weird that Albert keeps a picture that his mother had made for his father as a romantic gesture note and explains why he has such a massive Oedipus Complex in Gankutsuou.
- In The Dresden Files, this becomes a Running Gag:
- In Blood Rites, between Thomas Raith and Harry Dresden, made worse by the fact that 1) they cannot openly reveal their true relationship, and 2) they were sharing the latter's small apartment for an extended period after Blood Rites. And 3) Thomas was deliberately pretending to be gay in his public life for completely unrelated reasons. Considering all this together, we should have seen it coming from the paragraph after that Wizard of Oz poster on Thomas's wall.
- In Dead Beat, Waldo Butters (after meeting Thomas for the first time), is hurt that Harry hasn't previously come out to him. Thomas deliberately kisses Harry on the head and pours him tea after listening to Harry's subsequent startled denial.
- In White Night, this is deliberately exploited to explain to building security why Harry has a key to Thomas' apartment and has turned up there in the latter's absence.
- Roger Ebert listed a variant of this trope in his analytical book Ebert's Glossary of Movie Terms as the "Inevitable Sister".
In any movie where the heroine catches her boyfriend dancing in public with another woman and makes a big scene, the other woman invariably turns out to be the boyfriend's sister. Cf. MYSTIC PIZZA, etc.
- Forest Kingdom: In the Hawk & Fisher spinoff series' Book 3 (The God Killer), when investigating the members of the God Squad, Fisher tracks one of them, Charles Buchan, to the base of the Sisters of Joy, who are basically a religion dedicated to sex and pleasure. She assumes he's there to partake of what they offer, until he admits the truth — the woman he loved had died giving birth to their illegitimate daughter Annette, who's grown up as one of the Sisters, and he only recently found out about her.
- Fox Demon Cultivation Manual: Inverted. Because Song Ci drank Rong Bai's blood, his own blood smells like Rong Bai's. This prompts Zhu Yishu and (briefly) Rong Bai himself to assume they're related. The misunderstanding is cleared up long before they start a relationship, though.
- In the first book of The Heroes of Olympus, Piper, who has a crush on Jason, briefly wonders if the person in the photograph he's looking at is an old girlfriend he's just starting to remember. The audience already knows, and Jason later clarifies, that the photo is of the sister he recently found out he had.
- Keeper of the Lost Cities: When she first meets Tam and Linh, Sophie wonders if they're boyfriend and girlfriend, since they're sitting close together and sharing a fruit. It is only once she sees their faces that she realizes they're brother and sister.
- Isaac Asimov's "Liar! (1941)": Herbie, a telepathic robot, tells Dr Calvin that the man she's attracted to isn't dating. In fact, The 'other woman' that she had seen was a first cousin, and not his girlfriend. Subverted, as he is planning on marrying her soon after solving the problem of telepathic robots.
- The Moving Finger: The protagonist and his sister move to a rural English village. When a round of anonymous letters is sent to people in the village, theirs accuses them of being an unmarried couple posing as siblings (it doesn't help that they don't look related, one taking after their father and the other after their mother).
- Nina Tanleven: Unusual variant in The Ghost in the Big Brass Bed - Nine goes to Phoebe Watson's home to feed her cat while Phoebe is in the hospital, and while she's upstairs, a man also unexpectedly enters the house. Nine avoids being seen by him but does hear his voice and, afraid that he's a burglar, manages to sneak out in time to meet up with her father, get home and call the police. The next day, while she and Chris are in the hospital visiting Phoebe, the "prowler" also turns up to visit and turns out to be Phoebe's cousin Byron, whom Phoebe didn't expect to arrive until that morning. Fortunately, Byron is somewhat amused by the misunderstanding after Nine explains herself, and accepts her apology; it helps that Phoebe keeps his picture on her dresser, which he was able to use as proof that he really did belong there when the police came by.
- North and South by Elizabeth Gaskell. Similarly, the heroine Margaret Hale's reputation is damaged in the hero's eyes after she is observed saying an intimate goodbye to a young man at a train station, and then lies about it. She cannot reveal that the young man was her brother as he is a wanted man.
- Averted in Northanger Abbey when the heroine sees her Love Interest with another woman on his arm and immediately comes to the correct conclusion that it's his sister, thereby depriving herself of a perfect opportunity for needless drama, as the narrator tells us.
- In one of Anne Perry's Late Victorian mysteries, a married man is seen with a pretty young woman who he obviously cares for very much. It turns out she is his daughter, his legitimate daughter, as his second marriage is bigamous and his four children by it are illegitimate. Ouch.
- Rob Roy: Frank sees Diana riding with a strange man, and he assumes that man must be her lover. Later, Frank learns he was Diana's father, Sir Frederick Vernon.
- In The Schoolmouse by Dick King-Smith, Flora's boyfriend Boy mistakes her father Robin for her suitor and bites through his paw. In a case of poor communication leading to fights, Robin doesn't outright say that he's Flora's father when asked by Boy, simply saying that he's come to see Flora.
- The Secret History: When Richard first sees Charles and Camilla together, he assumes they are boyfriend and girlfriend. He later learns that they are twins. Even later he learns that he was still right the first time.
- Special Operations: In the first book, Loki is jealous of a British commando who parachutes into the area and who Frey clearly knows and cares about. He turns out to be her older half-brother (who was sent into the mission because of his family ties in the area).
- The Tenant of Wildfell Hall by Anne Brontë. The hero, Gilbert, becomes suspicious of Helen's close relationship with her landlord Mr. Lawrence and knocks the poor man unconscious in a fit of jealous rage. Of course, Mr. Lawrence is Helen's brother. (As she's living under a false identity to evade pursuit by her alcoholic husband, they have kept their relationship secret, but their clandestine meetings have fueled gossip in the village.)
- In Walk Two Moons, after Phoebe's mother leaves on an unexpected trip, Phoebe and Sal track her down and catch her with a guy in his late teens, who she's kissing on the cheek. Phoebe, who saw the guy earlier and thought he might have been a deranged killer, is not happy. When her mother returns, she brings him along and explains that he's her son from a previous marriage who she recently was reunited with. Sal also notices that her teacher, Mr. Birkway, and Mrs. Cadaver, the woman she and her father live with, spend quite a lot of time together and wonders if the two are in love. It turns out that they're brother and sister.
- Warrior Cats: Fireheart and Sandstorm encounter a strange kittypet she-cat who starts fawning over Fireheart and licking him furiously. Sandstorm wasn't Fireheart's mate but had feelings for him. Sandstorm is unnerved by the she-cat's affection, however Fireheart quickly mentions that she's his sister, Princess.
- E. Nesbit's The Wouldbegoods: This trope is cheerfully lampshaded. The child protagonists are reluctantly trying to reunite their adult friend with his long-lost sweetheart even though they fear this will mean he has less time for them. So they are pleased, rather than scandalised, to find the girl embracing another man. But their relief is short-lived and the narrator reflects, "He might have known it was her brother, because in rotten grown-up books if a girl kisses a man in a shrubbery that is not the man you think she's in love with; it always turns out to be a brother."
- 45 RPM: Maribel mistakes Robert's sister Clara for his ex-girlfriend when she first sees them together.
- In Black Books, Fran gets annoyed when she sees her boyfriend having coffee with another girl, that turns out to be his sister. Partially lampshade hung when Bernard guesses, correctly, that that was the case before Fran actually reveals that to him. They break up anyway, but only over of how Fran jumped to conclusions and behaved childishly. Her boyfriend comments that he wants to date someone less "mad".
- Boardwalk Empire: New York gangster "Lucky" Luciano and Atlantic City gangster Jimmy Darmody quickly develop a mutual dislike of each other. So when Luciano meets a stunning redhead who introduces herself as "Mrs. Darmody", Luciano quickly makes the moves on her, assuming she's Jimmy's wife. When his boss Rothstein calls him up, he's quite amused, telling Luciano he didn't bang Jimmy's wife, he banged Jimmy's mother. Said mother later even helps out her boy with luring Luciano into a trap.
- Used secondhand in Bones, where Booth and Brennan saw Sweets' girlfriend in a wedding dress hugging another man, and after some disagreement Brennan told him, made worse by her directly informing him that his girlfriend was engaged to someone else. Luckily for them when he confronted her about it she explained what had actually been happening (she was trying on a wedding dress for her cousin, who's the same size), and they... reconciled.
- ''The Carol Burnett Show": In one skit, Carol and Vicky play friends having lunch in a nice restaurant, when Carol spots Vicky's husband (Harvey) and a younger woman seated at the table behind Vicky's back. Assuming he's cheating, Carol does everything in her power to distract Vicky from noticing, including tripping their waiter, played by Tim Conway, no less than three times as Harvey walks past their table. Harvey finally spots Carol and calls her & Vicky over to his table to introduce Carol to his "much younger sister," much to Carol's chagrin.
- Played with a lot in Castle, where Rick says to Beckett more than once things like 'I've got a big date' or that he has a lady to see when he's actually just talking about his daughter Alexis. As a Genre Savvy writer, he does this deliberately to feed the Unresolved Sexual Tension and make Beckett jealous, which she does a poor job of hiding for someone trying to convince everyone (including herself) that her relationship with Rick is strictly professional. It also shows up in a more conventional form in one episode where a famous baseball player's wife suspects him of an affair after he's seen with an attractive young(er) woman — the younger woman is actually his daughter from a previous relationship, who assures the woman (technically her stepmother... of sorts) that her father really did love her.
- Exploited and Played for Drama in the Cold Case episode "Thick As Thieves". As the detectives eventually find out, the Victim of the Week and her boyfriend were a team of con artists who claimed to be a couple as part of their swindling strategy, when the truth is that the "boyfriend" was the victim's son.
- Tracy Barlow invokes this in Coronation Street when her brother Peter is upstairs as her abusive boyfriend Charlie arrives home. Tracy pretends that she has brought a guy home but quickly shouts that it's her brother when Charlie rushes upstairs. Unfortunately, he finds Peter shirtless (he'd just had a shower) and beats him up.
- In Desperate Housewives, Lynette pretends to be a teenage girl in a chatroom so she can have real conversations again with her son. Unfortunately, he gets a crush on her online persona and she has to let him down easy with a goodbye message...which she signs "Love, Mom."
- In an episode of Drake & Josh, Josh sees his ex-girlfriend Mindy hanging out with and being affectionate with what he assumes is a new boyfriend. Of course, he turns out to be her cousin. When Josh asks why Mindy didn't say so in the first place, she says that Josh wasn't letting her explain.
- Firefly: When the box containing River is opened, Mal says scornfully, something to the effect of "Is she your lover, because you do seem awfully worried." Specifically, he accuses Simon of human trafficking, then sarcastically says something like "Oh, I'm sorry. Is this one for you, is it true love?"
- An episode of Frasier had Niles shopping for an engagement ring, accompanied by Frasier and Martin. Everyone in the store assumed the brothers were getting engaged to each other and applauded, much to their father's dismay. Seeing as how they do everything together, including going to their high school prom, this has probably happened to them a lot.
- Used in The Fresh Prince of Bel-Air, in which Hilary and Carlton were each flirting (separately) with an online admirer. They went to the restaurant to meet their date... only to discover that it was each other.
- Rachel is shocked that her boyfriend seems to be cheating on her, only to find that the woman he was hugging was his sister. Then Rachel goes to their apartment and finds them tickling each other, eating whipped cream from each other's laps and, ultimately, sharing a bath together. Her boyfriend insists that it's all innocent and just proof that they're close enough as siblings to behave as they did as kids, but Rachel thinks it's too weird and leaves him.
- Siblings Ross and Monica Geller, in spite of never having on-screen moments that may confuse people, are horrified to learn in the tenth season that they had actually kissed while teenagers. (It was dark, Ross thought he was kissing Rachel, Monica thought he was her "Midnight Mystery Kisser.")
- A brief bit in "The One Where They're Up All Night" where Rachel sees a picture of a girl on her assistant/boyfriend Tag's desk and says "Who's this chippy? Seems a little young for you, but whatever." She's his sister.
- Inverted in an episode where Joey, trying to get back together with an ex-girlfriend, arranges for a double date by telling his ex that Monica is his girlfriend while pretending to Monica that he's got a date with his ex who will be bringing her brother along as a date for Monica. Monica grows increasingly uneasy over the course of the evening at the behavior of the people she thinks are siblings. She figures it out when she sees the guilty look on Joey's face.
- Full House had an episode where Kimmy and Stephanie see DJ's boyfriend Steve with another woman, but the woman was later revealed to be his cousin.
- Gilmore Girls
- When Richard is in the hospital, Michel hijacks Sookie's concerned phone call to inform Lorelai that a "couple" who checked into the Dragonfly Inn is actually a brother-sister pair.
- In Season 6, Zack, who has recently broken up with Lane, is jealous when he sees her hanging around a Korean guy and confronts her mother about it. It turns out the guy is a young uncle.
- Subverted in Good Luck Charlie: Teddy sees Spencer with a girl named Skylar and confronts him about it, only for Spencer to inform her that Skylar's actually his cousin. He's lying — he's dating Skylar on the side and that Skylar, who had no idea that Spencer already had a girlfriend, was under the impression that Teddy was his cousin. When the two girls eventually realize the truth, neither one takes it well - -especially Teddy.
- In one episode of Brazilian TV show A Grande Família (The Big Family), Isabel "Bebel" Carrara assumed her husband was having an affair. Turns out he was just meeting his long-lost sister.
- Grey's Anatomy introduced Patrick Dempsey's sister this way.
- How I Met Your Mother featured an extended flashback to Barney's first love. His girlfriend was supposed to go into the Peace Corps with him, but bailed at the last moment with the excuse "My father won't let me go." Barney goes to give the "father" a piece of his mind and walks in on them kissing.
- In Keeping Up Appearances, when Hyacinth sees Emmett, her neighbour Elizabeth's brother, for the first time, she believes her to be cheating on her absent husband. Although it isn't because she sees him with her, it's because she sees him at her house, dressed only in a towel.
- Malcolm in the Middle:
- Francis' wife Piama is meeting with a man from her past that she doesn't want to introduce him to. Francis' friend convinces him it must be her recently paroled ex-husband and they're probably hooking up, causing Francis to storm home and confront them. Turns out the man is Piama's estranged father looking for money, and she was just planning on chewing him out and turning him away while Francis was at work.
- Inverted in when Reese walks in on his wife cheating on him and she tries to pass off her lover as her brother. Having turned into an Extreme Doormat by then, Reese buys it, and even continues to call him her brother when he later walks in on the two of them in bed.
- In one episode of Mighty Morphin' Power Rangers set after the marriage of Lord Zedd and Rita Repulsa, Rita greets a man whom she actually seems happy to see. Zedd is (to his own surprise) rather perturbed by this until Rita introduces the new arrival as her brother, Rito Revolto.
- In the Murder, She Wrote episode "The Grand Old Lady", a married soldier coming home from World War II had a relationship with his nurse, who accompanied him back to the US because she couldn't bear to leave him until she had to, but intends to remain on the ship and go straight back to Britain once he's reunited with his wife. Once the murder's solved and everyone's finally able to disembark, she sees a woman running up to him and embracing him, and turns away. The next time the soldier and nurse appear, they're together, with the soldier cheerfully explaining that his sister met him at the docks to tell him his wife had run off with a lumberjack.
- One episode of Power Rangers Lightspeed Rescue had Ms. Fairweather call a famous scientist named Clark to help her build some armour for the Rangers. They get along exceedingly well, and Clark is even given the honor of revealing her first name (Angela) to the audience. Meanwhile, Joel, who was in love with her, is seen getting very, very jealous, and even tries to sabotage one of their 'dates.' Finally, they get attacked by a Monster of the Week and, seeing Ms. Fairweather's obvious distress when Clark is still in danger, Joel decides he wants his beloved to be happy and saves his rival. Later, he gamely tells her how glad he is for her and that she deserves the best boyfriend ever. Naturally, this is when he is treated both to the reveal that he was her brother all along and to a kiss on the cheek. In a No Hugging, No Kissing series, even. Joel and Ms. Fairweather have the distinction of being the first PR Official Couple to get married.
- Remington Steele: Steele once saw Laura talking with her brother-in-law and thought she was having an affair. It didn't really help that the brother-in-law kept kissing her on the cheek...
- In Reno 911!, the sheriff department notice Dept. Garcia embracing a beautiful woman. They're all happy for him for finally finding somebody, but then later they see that same girl with another guy. They all decide to do Garcia a favor and run the guy out of town. The next day, a depressed Garcia comes into work and tells everyone how his sister's fiancé left without any warning and that she's very upset over it.
- In Sabrina the Teenage Witch, Hilda and Zelda go to Parent-Teacher Night, and Zelda is attracted to Mr. Pool. After introducing themselves as Sabrina's aunts, she quickly adds that they're sisters, "not an alternative couple."
- Schitt's Creek: In Season 1, Alexis thinks her crush Mutt is having an affair with the mayor's wife, Jocelyn. Turns out Mutt is Roland and Jocelyn's estranged son.
- Supergirl (2015):
- When Supergirl surfaces for the first time, people wonder whether she's somehow linked to Superman. Kara Danvers visibly cringes when she listens to her boss suggesting they may be lovers.
- Cat's emails get hacked and Kara, Jimmy, and Winn are tasked with going through them to find anything potentially scandalous. They find evidence that Cat has been wiring money monthly to some unknown twenty-four-year-old man. When Kara brings this up to Cat — certainly not suggesting that she might be paying for a male escort or something! — Cat explains that he's her estranged older son.
- Kara and Alex have been mistaken for a couple at least once. They're foster sisters.
- The Winchester brothers in Supernatural are frequently mistaken for a gay couple. This apparently does not stop being funny, ever. In this Crapsack World, you take laughs when they come.
- Taboo: Subverted when Lorna Bow sees James Delaney paying a lot of attention to Zilpha during an upper-class party they were invited to. The two women later run into each other in the ladies room, and Lorna inquires of Zilpha whether she should consider her a rival for James's affections. Zilpha defiantly states she has no cause to worry since she and James share the same father. The subversion results from the fact that the two half-siblings actually used to be lovers as teenagers and still share a large amount of sexual tension.
- In True Blood, Violet Mazurski assumed Jason Stackhouse was cheating on her when she saw him hanging out with Sookie until he told her Sookie is his sister. Violet's response is to say, "Then you're my sister too!" and then kiss her on the lips.
- In The Tudors, Henry spots Anne talking with another man; later, during an illicit meeting between them, wraps a hand around her throat and demands to know who he was. As Anne explains, and the audience already knows, the other man was her brother, George.
- Two and a Half Men features brothers Charlie and Alan acting a lot like a married gay couple, which is a running gag in several episodes. For instance, on one occasion Alan's out camping, Charlie's asked by a friend about "the wife and kid."
- In The Umbrella Academy, in Episode 8 the siblings are hiding out at a children's bowling alley. Luther and Allison head outside to deal with another issue, leaving Diego and Klaus to watch over Five (actually the oldest of the brothers but currently stuck in a young teenage body.) A friendly mother approaches them and:
Mother: Excuse me, it's my son Kenny's birthday today, and wouldn't you be happier playing with kids your own age? Assuming it's OK with your dads.
- In the last season of Veronica Mars, Mac's newly introduced animal-rights-activist love interest turned out to have a female housemate, who triggered this trope.
- The Vicar of Dibley:
- Geraldine falls in love with Harry and gets jealous when she sees him walking around the village with another woman. It isn't until he shows up at the vicarage to propose that she finds out the woman is his sister.
- Variant: An earlier episode had Geraldine nearly call off Alice and Hugo's wedding when she thought that David Horton, Hugo's father, was also Alice's dad via a one-night stand. Turned out the David in question was a relative of the show's David who had the same name.
- In the Child Ballad "Child Maurice" or "Gil Maurice", the titular character is murdered by a jealous lord whose wife he arranged to secretly meet in a forest. The wife then confesses to the lord that he was not her lover, but an illegitimate son that she bore in secret before their relationship.
- A darker variation occurs in The Tailor Shop on Enbizaka from the Vocaloid series Evillious Chronicles. The tailor sees her lover with three different girls on different occasions, much to her annoyance. It turns out that her "lover" never even met her before and the three girls were his wife and two daughters. Who the tailor all murdered in a jealous rage, including the man as well.
- This is the plot of "After the Ball." The narrator found his sweetheart being kissed by another man and wouldn't listen when she tried to explain it's Not What It Looks Like. Only some time later, after she had died, did he learn that the man was her brother.
- The Gospel duo of Bebe and CeCe Winans could come across this way to people who don't know they're siblings. For siblings, they're very close, and mixed-sex duos that're married are way more common than siblings. It also doesn't help that they sing a lot of God-Is-Love Songs.
- Chuck Berry's song "Memphis, Tennessee" is built on getting the listener to make this error, and then surprising them. He sings about a girl he misses, and how they were parted because of her mother, and he's trying to get in touch with her. In the second-to-last line, he sings "Marie is only six years old" and suddenly you realize Marie is his daughter, not his Love Interest.
- Happens in The Space Gypsy Adventures episode, Damien's Valentine's Day Panic. Jehlise sees Damien talking with another girl in the spaceport cafe, and assumes that Damien has dumped her in favor of 'his own kind' (the girl is a young fox with 'raccoon markings', like Damien). When she confronts him about it the next day, he explains that the girl is in fact his cousin Anita.
- A comic in MAD had a man see his wife walking down the street with another man and immediately assume she was cheating on him. When he confronts her about this, she explains (through a picture of them as babies in a carriage together) that they are actually siblings.
- In the Gilbert and Sullivan operetta Iolanthe, the twenty-five-year-old protagonist Strephon is caught embracing a young woman (actually his mother). Since his mother is an immortal fairy and, as such, appears to be about seventeen, Strephon's fiancee is understandably skeptical and promptly breaks off their engagement.
- In the opera Rigoletto, the courters learn that Rigoletto secretly has a young woman living in his house. They assume she's his mistress, kidnap her and give her to the lecherous Duke to humiliate the hated jester. As it turns out, she's his innocent, virginal daughter, and his response to her kidnapping isn't embarrassment, but vengeful Papa Wolf rage.
- Inverted in Groovelily's Striking 12 in the imaginary party sequence. The protagonist can't bring himself to tell an acquaintance he's broken his engagement to Diane, so when Diane arrives with another man, the acquaintance assumes it's her brother. She ends up saying, "I don't think I've ever seen two siblings kissing quite like that..."
- In Fate/Grand Order, Miyamoto Mushashi teases Fuuma Kotarou for sneaking glances at Katou Danzou, thinking he has a crush on her. He later explains to Musashi that Danzou was his adoptive mother.
- This happens when Chrom meets Lucina in Fire Emblem Awakening. Whoever his wife is at first assumes he's having an affair. Turns out Lucina is their Kid from the Future.
- In JoJo's Bizarre Adventure: Eyes of Heaven, Narciso Anasui/Narc Anastasia is aghast when he finds his crush, Jolyne, talking to a young guy he doesn't recognize. Turns out it's a younger version of Jotaro Kujo... Jolyne's future dad, something that also takes Jotaro by surprise.
- In Persona 5, one of Yusuke's confidant events has him and Joker go to the park on a rowboat to find inspiration for his art. Yusuke ends up drawing a man and woman in a rowboat next to theirs, believing them to be a couple in love. They notice him drawing them, and they clarify that they're just brother and sister.
- In an official video for Pokémon Super Mystery Dungeon, two Jellicent interview Umbreon and Espeon on their relationship. The female Jellicent is convinced they're a couple and try to get the two to admit to their relationship. Umbreon and Espeon skirt around the issue, and Jellicent never finds out if they're dating or not. In the previous title the two were shown to be siblings, though the fact is never shown in Super Mystery Dungeon.
- An ironic one comes up early on in Daughter for Dessert. Moe Mortelli, on seeing Amanda, thinks shes the protagonist's girlfriend. If only he could see what they would eventually get up to....
- In My Candy Love, upon the player character's first meeting with Amber, she is told to "stay away from Nathaniel." She then goes to talk to Nathaniel and has the option of warning him about a girl named Amber who likes him. Since Amber is his twin sister, he's a little ticked off (although to be fair, the player had no way of knowing that).
- Zero Escape:
- Inverted in Virtue's Last Reward. When Sigma sees a young girl on a picture Tenmyouji has, he mistakenly thinks that the girl is Tenmyouji's granddaughter. Quark then clarifies that the girl was Tenmyouji's first love. Tenmyouji denies. Sigma doesn't buy his denial and calls him "hopeless romantic".
- In Zero Time Dilemma, when Carlos talks about Maria, Junpei asks if Maria is his girlfriend. Maria is Carlos's sister.
- Badly Drawn Kitties used this once. Lucy overheard Lydia's boyfriend Dirk and another woman declaring their love for each other and assumed because of the woman's foreign accent that she wasn't related to Dirk. It turned out she was in fact his sister and she'd just been studying in England, and so had picked up the accent.
"And you're not sleeping with Dirk?"
"Uh, no. I said England, not Georgia."
- Given that Lucy had had a "unique relationship" with her own brother, that wasn't actually an unreasonable assumption on her part. But in the original continuity, Lucy and her brother were originally from Georgia (the US state), so there may have been some Lampshade Hanging in the rebuttal.
- There is a meta version in the fandom of Housepets! Some believed that Cosmic Chess Game combatants Pete and Spirit Dragon may have been ex-partners, or even had some present Belligerent Sexual Tension going on. It was found out at the end of the climax arc "Heaven's Not Enough" that they were actually brother and sister.
- In Kevin & Kell, a well-intentioned Kell sets her friend Aby Eyeshine up on a blind date with Rudy's hunting coach, Sam Pardus. Sam and Aby, in accordance with the trope, are brother and sister, but Kell didn't know that (all parties found the whole thing amusing). Sam being disguised as a leopard (while Aby is a house cat) and changing his last name in order to coach the Caliban Academy team despite being domesticated makes it that much harder for Kell to find out.
- Ménage à 3: Zii makes this mistake, then, panicked, tries to cover herself under it. (Zii is new at monogamy.) Her boyfriend is sweetly understanding.
- In Savestate, brother and sister Kade and Nicole have been mistaken for a couple on at least two occasions.
- Done in Too Much Information (2005), when the two main girls of Ace's Unwanted Harem sees him driving a pretty, blond girl around in his sports-car... it's actually his cousin, whom he's giving a ride to an abortion clinic so her parents won't find out.
- Inverted in one story on Not Always Right; the two people are a couple but are mistaken for siblings. And in another story where a man who is mistaken for a young teenage boy wants to buy an M-rated video game, and when his wife comes in to tell the clerk that he's allowed to buy that game, she is mistaken for his mother and told that the game may not be appropriate for her son.
- A nosy lady at a restaurant on Not Always Friendly tries to shame a man for getting dinner with a girl young enough to be his daughter. The man replies that the girl IS his daughter, shaming the lady instead for making assumptions.
- The Amazing World of Gumball:
- In the episode "The Flower" Gumball is jealous of Leslie because the latter has been hanging out with Penny more, but it turns out that Leslie is Penny's cousin—it should also be noted that Leslie is Ambiguously Gay.
- "The Ad" has the Wattersons hatch a plan to make extra money by renting their house to an elderly goat couple, Bernie and Ethel, while still secretly living in the house. When Bernie and Ethel are accidentally sent into a fainting spell (turns out they're fainting goats), the family stages photos of them at various vacation spots in a Weekend At Bernies-esque montage. The plan almost works... until Bernie and Ethel notice a photo of "them" on a romantic date at the burger shop. Turns out they're brother and sister. Oops.
- Arthur: In "Mr. Ratburn & the Special Someone", the kids learn that Mr. Ratburn's engaged to get married. When they run into him in a local diner with a woman named Patty, they assume that she's soon-to-be wife, and based on her abrasive personality, they try to sabotage the wedding for his own good. Not until the wedding itself do they learn that she is in fact his older sister, and the officiant — it also turns out that Mr. Ratburn's marrying a mannote and not a woman.
- In the Bob's Burgers episode "Boywatch" Tina becomes Shipper on Deck to two of her fellow trainee lifeguards.
Tina: So, what do you think of Jason? I'm pretty sure he's into you.
Willow: Jason's my brother.
Tina: Oh. I was wondering why you both said "Bye Mom" to the same lady. Still, there's something there.
- In the Camp Lazlo episode "Spacemates", Chip and Skip manage to do this to themselves.
Chip: Are you my soulmate?
Skip: No, I'm your brother.
Chip: ...Oh. This universe is so confusing.
- Captain Planet and the Planeteers: In the episode "Missing Linka", a disguised Wheeler helps saves Linka's life in her hometown in Russia. He initially assumes that a guy named Mishka is her boyfriend, but in the end, Linka reveals that Mishka's actually her brother.
- In Codename: Kids Next Door, Nigel's girlfriend, Lizzie, discovers him with another girl (an operative who goes by "Numbuh 10"). Thinking they're on a date she confronts him about it creating a huge scene. Nigel and Numbuh 10 explain that they had actually recently discovered that Nigel's mom and Numbuh 10's dad are brother-and-sister, which makes them first cousins. Instead of being relieved, Lizzie gets disgusted, still believing that it's a date.
- Inverted on Daria — when Daria first starts dating Tom, Kevin and Brittany initially assume that he's her brother. It seems to be partially because they think it's weird for someone like Daria to date, and partially because they know Tom was recently involved with Daria's friend, Jane.
- A newscast in DC Super Hero Girls calls Supergirl the "kid sister" of Superman. She's actually his cousin (and his older one, at that).
- Family Guy:
- Brian is trying to build a friendship with Quagmire, who he recently discovered actually hates him. At one point he sees a disheveled woman in Quagmire's doorway and, knowing the kind of guy that Quagmire is, starts making jokes about them having apparently just had sex. Quagmire angrily informs him that the woman is actually his sister who came over to escape her abusive boyfriend.
Quagmire: I also have a deaf brother! Want to make fun of him, too?!
- He makes a similar mistake in "The Thin White Line" while making awkward small talk with his psychiatrist:
Brian: I, uh, I notice you got a new receptionist. Nice little body on her, huh?
Dr. Kaplan: That's my daughter.
Brian: ... We could probably call this an early day, huh?
- Brian is trying to build a friendship with Quagmire, who he recently discovered actually hates him. At one point he sees a disheveled woman in Quagmire's doorway and, knowing the kind of guy that Quagmire is, starts making jokes about them having apparently just had sex. Quagmire angrily informs him that the woman is actually his sister who came over to escape her abusive boyfriend.
- In Garfield and Friends, "Wedding Bell Blues", Jon's cousin, Marian, in town for her upcoming wedding, comes over to visit her cousin, whom she hasn't seen in years. Marion explains to Jon that her soon-to-be husband's name is "Harold John," and when she points out the coincidence, she'll be "Marian John" (say it out loud). Garfield and Odie are standing outside the window of Jon's house when they hear Marian say this, but (not realizing that she and Jon are cousins) misinterpret their exchange as Marian being Jon's fiancée. Garfield soon gets worried that Jon won't pay as much attention to him after he gets married, as well as worrying that Jon's potential future children will play harmful games with him. In the end, he discovers what the viewer knew from the beginning: Marian is Jon's cousin and that she's marrying a man whose last name happens to be "John." His attempt to disrupt the wedding is fortunately taken in good humor (Harold in fact thinks the whole thing is hilarious) and the wedding continues.
- Jem: Aja and the others make this error in "Britrock". Aja's boyfriend starts acting more quiet than usual and tells her he has to tell her a secret he's been keeping. When Craig tells Aja Stormer is his secret Aja gets mad and it only gets worse when he introduces himself as "Craig Phillips" and Stormer as "Mary Phillips". Even Craig's bandmates lampshade how badly Craig went about the situation and decide to leave the room. Aja slaps Craig and runs away crying. It isn't until a moment later that Kimber informs her Stormer is Craig's little sister, not his wife. Craig had been hiding the fact because The Misfits and The Holograms are bitter enemies. Apparently, the fact Craig and Stormer are the only characters with deep blue hair went unnoticed by Aja.
- Les Sisters: Marine sees Maxence with a girl at the park and assumes that he's cheating on Wendy. Near the end, Marine and Wendy find out what the audience knew from the start, that she's only his cousin.
- My Little Pony Tales: In the episode "Sister of the Bride," Clover's sister, Meadowlark, is getting married to Cheval, and at one point in the episode, Clover and her friends see Cheval seemingly flirting with another girl, and they immediately accuse him of cheating on Clover's sister. Eventually, Cheval explains that the girl they saw him with was actually his sister.
- Next Avengers: Heroes of Tomorrow: of the main characters, four were raised together by Iron Man, while Francisnote was not, leading to this exchange:
- In the Phineas and Ferb episode "De Plane! De Plane!", Candace becomes jealous when she finds her crush, Jeremy, seemingly flirting with another girl, who later turns out to be his cousin — Jeremy explains that the conversation Candace overheard between was them was Nicolette (Jeremy's cousin) telling him about this video game level where she was stuck on, and how it took her at least a week to get out of it and move on to the next level.
- Regular Show: Mordecai sees a man bending down in front of his crush, Margaret, and overhears her saying "Yes dude, yes!" Thinking the man was her fiancé, Mordecai spends the episode in a depression. In the end, it turns out that the guy (named John) is actually Margaret's cousin, and that the exchange Mordecai witnessed was actually just John showing off his new prosthetic leg.
- In the last episode of Road Rovers, Hunter tracked down and went to visit a female dog from his past. Everybody, particularly Colleen, thought she was an old girlfriend. Turns out she was his mother.
- Rocko's Modern Life. In "S.W.A.K." Rocko is smitten with his new mail-carrier, Alicia, of which Heffer remarks Wallace from Pizza Face Pizza wouldn't be happy about. Rocko then spends the rest of the episode trying to keep Heffer — who had taken over Alicia's route while she took a sick leave - from delivering a love letter he wrote to her, in case it falls into Wallace's hands... which it does. Later, though, Rocko receives a response from Alicia, who notes that Wallace is her brother, much to his relief.
- The Simpsons:
- Lisa has a crush on an older boy, then hears him having a phone conversation with a girl, saying that he can't wait for her to get there. When the girl arrives, Lisa purposefully gets her lost in the woods, only to find out that her "rival" is, of course, the boy's sister. The boy, naturally, does not take this well.
Lisa: Sister! You mean she's not your girlfriend?
Luke: Hell, no. They outlawed that in this state two years ago.
- There is a variation in the episode "Bart vs. Lisa vs. The Third Grade"; some guys make fun of Bart by saying Lisa is his girlfriend after they see them holding hands. To be fair, the guys probably thought that Bart and Lisa were just unrelated friends/classmates instead of siblings.
[Bart and Lisa are holding hands]
Guy #1: Hey, dude, who's your girlfriend?
Guy #2: Did you see his face?
[Bart and Lisa aren't holding hands]
Guy #1: Well, what happened, dude? Did your girlfriend dump you?
- Lisa has a crush on an older boy, then hears him having a phone conversation with a girl, saying that he can't wait for her to get there. When the girl arrives, Lisa purposefully gets her lost in the woods, only to find out that her "rival" is, of course, the boy's sister. The boy, naturally, does not take this well.
- Steven Universe: The Crystal Gems are Steven's guardians and Parental Substitutes, but Mayor Dewey assumed they were his sisters. Steven's father's cousin assumed Garnet was his biological mother. In a subversion, Sadie avoids assuming a specific relation, and simply calls them "Steven's... family?"
- In Voltron: Legendary Defender, Pidge has a picture that the others assume to be of him and a girlfriend. It's actually a picture of Pidge and her brother.
- In an episode of The Weekenders, Tish requests that she just hang out by herself for the upcoming weekend... and the others spy on her, suspecting ulterior motives, and see her hanging out with a boy her age all over town. When they confront her about her 'new friend,' she reveals first of all that it's her cousin Doug, and secondly, she had planned on being by herself that weekend, but Doug's visit was last-minute and her parents asked her to show him around.
- In an episode of X-Men: Evolution, Mystique brings Wanda into the Brotherhood house and Pietro has a huge Oh, Crap! moment when he sees her. During her rampage around the house, Toad asks if she's an ex-girlfriend. Pietro responds, "Worse! She's my sister."
- Actor Samuel L. Jackson was once photographed going to dinner with a much younger woman, which caused some speculation amongst celebrity gossipers. This eventually reached Jackson's mother, who called her son to see if this was true. His response? "You crazy woman, don't you recognize your own granddaughter?!"
- Ex-England football team coach Sven Goran Eriksson was snapped with a young blonde. The Daily Star "newspaper" ran a front-page story, basically emphasizing the fact that she was young, blonde and worth a few "Phwoar!"s. Just like the outcome of the above example, this woman was eventually revealed to be his daughter and not his girlfriend.
- The "Kiss Cam" at any sporting event makes this mistake every once in a while. Including one relatively well-known picture where a man actually holds up a sign with an arrow pointing to the woman he was sitting with and the text saying "My Sister" when the kiss cam focused on them. Although everyone, including his sister, seemed to think it was Actually Pretty Funny. Here's the story behind the guy with the sign.
- Richard and Karen Carpenter of Carpenters sang romance songs together and shared the same surname, so more than a few people in the media thought they were a married couple. They were brother and sister.
- Inverted for Helen Slater and Christian Slater. For years, thanks to them playing siblings in the movie The Legend of Billie Jean, the former has had to constantly tell curious fans that they're not actually brother and sister.