Fran: I spent some time explaining why he was a scumbag, and the fancy lady he was with was a slut.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. Oftentimes, the one who catches the two together is a lover or love interest of one of the siblings, and thus thinks that he/she is cheating or doesn't like her/him. Drama and/or Hilarity Ensues. 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. 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 a Relationship Writing Fumble. Sometimes though, it happens to be Exactly What It Looks Like. Warning: Spoilers Ahead.
Bernard: It was his sister.
Fran: [sullen] ...yes.
Bernard: It was his sister.
Fran: [sullen] ...yes.
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- In one of the many commercials John Stamos 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 woman so quickly, smashing a picture. He informs her that the other woman is actually his sister and that the picture she smashed was actually of their mother.
Anime and Manga
- 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?!
- Used as the final reveal in the Di Gi Charat Winter Garden spinoff.
- 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.
- 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.
- Used in Dragon Ball Z: Krillin mistakenly believes that Android 17 and 18 are a couple rather than Half-Identical Twins.
- 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."
- 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.
- Betrayal Knows My Name: Toko and Tsukumo Murasame seem to fit this, even going so far as to sleep in the same bed.
- 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.
- 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.)
- 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.
- A horrific one occurs in Elfen Lied, where Kaede mistakes Kouta's sister for his girlfriend. She promptly kills her and his father.
- It is possible to see this trope at work in Rebuild of Evangelion, counting Rei's affection for Shinji as purely maternal - if this be so, Asuka makes a fateful error.
- Invoked in Koe No Katachi. Shouya meets a boy who says he's Shouko's boyfriend. The awkward part 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.
- 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. 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.
- 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 bishonen younger brother, for Takeo. Ai described Takeo as very attractive looking however Takeo isn't what Oda expected.
- This, along with an extra serving of Mistaken for Gay, happens to brothers Cyclops and Havok in an issue of X-Factor.
- Batman had an example in the Golden Age, with the King of Cats, Catwoman's brother.
- In Star Wars: Invasion, Finn Galfridian assumed Jaina Solo and her brother Jacen were a couple.
- 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. At the end, when they're caught, a decidedly unimpressed Veronica informs them that the guy is her cousin.
- 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.
- Inverted 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?"
- 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 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.
- 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 with Kristoff, Sven, and Olaf to be together.
- 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.
- The plot catalyst in Gray Matters. The brother-and-sister team both kiss the same woman, too.
- 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.)
- 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.
- 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, "OMG! And I wanted ta--" before an explosion cuts her sentence off. Needless to say, she was squicked and befuddled by it all.
- Used in The Thomas Crown Affair (1999), 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.
- 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 Letters to Juliet, this causes Sophie some grief when Charlie introduces her to Patricia, his ex-girlfriend. It turns out that Patricia was also the name of his cousin whom he was really introducing.
- 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.
- This trope is a great staple of the literature of the 19th Century, as it allows the conflict and drama entailed by (usually) a woman flouting social norms, while still keeping the heroine innocent. Jerome K Jerome takes a moment to complain about theatre plays relying on this trope far too much.
- In Susan Calvin's introductory story, a lying robot persuades her that she's made this mistake about a love interest.
- 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 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' 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.
- 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.
- 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.
- 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 had been having an affair until he confesses his fears to Jo's brother - who is very quick to explain the truth.
- 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."
- 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 one of Kathy Reichs' Temperance Brennan novels, several other characters mention seeing her boyfriend Ryan going out with a young woman. Its eventually revealed that she is his recently discovered daughter.
- 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.
- 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!"
- 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.
- One of the stories of The Berenstain Bears had classmate Queenie McBear put an unapproved article in the school newspaper about a teacher being seen having coffee with a pretty teenager, causing a fuss and an inquiry for unprofessional behavior. Turns out his teenage niece had been visiting him.
- Inverted in Little Women. A suitor of Jo's calls at the house and is informed that "Ms March and her husband live next door". He assumes Jo has married someone else but Jo hurries down the road after him to explain that it's her sister who is married.
- 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.
- 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, his second marriage is bigamous and his four children by it illegitimate. Ouch.
Live Action TV
- 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".
- 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 (her trying on a wedding dress for her cousin, who's the same size), and they...reconciled.
- 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 others' 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, Rachel thinks it's too weird, and leaves him.
- Sibling Ross and Monica Geller, who in spite of never having on-screen moments that may confuse people, are shocked to learn in the tenth season that they had actually kissed while teenagers (in fact, Ross was Monica's first kiss ever).
- 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.
- Reversed 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.
- Grey's Anatomy introduced Patrick Dempsey's sister this way.
- 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, in one occasion Alan's out camping, Charlie's asked by a friend about "the wife and kid".
- Part of the solution in one House episode.
- 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.
- 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...
- 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.
- 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:
- Done with Geraldine's love interest in the final episodes.
- 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.
- 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?"
- 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.
- Full House had an episode where Kimmy and Stephanie see DJ's boyfriend Steve with a woman who turns out to be his cousin.
- 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."
- Inverted in Malcolm in the Middle, where 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.
- 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.
- 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.
- 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 a pretty young woman. Turns out it's his daughter from a previous relationship.
- 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.
- 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.
- 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 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, "Then you're my sister too!" and then kissing her on the lips.
- 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 is his cousin. He was lying. He was actually dating Skylar on the side and Skylar was under the impression that Teddy was his cousin. When they eventually realize the truth, neither one takes it well.
- 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."
- In an episode of 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.
- On Supergirl, 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.
- A darker variation occurs in The Tailor Shop on Enbizaka from Vocaloid. 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 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.
- 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. The coast isn't clear, though: Toni reveals that she does in fact already have a boyfriend.
- 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.
- 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.
- This happens when Chrom meets Lucina in Fire Emblem Awakening. Whoever his wife is at first assumes he's having an afair. Turns out Lucina is their Kid from the Future.
- 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.
- 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.)
- 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.
- 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.
- Done in Too Much Information, 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.
- 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.
- Captain Planet and the Planeteers: In the episode "Missing Linka", a disguised Wheeler helps saves the life of Linka in her hometown in Russia. He believes Mishka's an old boyfriend of Linka's, but it's later revealed that he's actually her older brother.
- In Garfield and Friends, "Wedding Bell Blues", Jon's cousin Marion, in town for her upcoming wedding, comes over to visit. She says the groom-to-be is named Harold John. When she points out the coincidence, and that, as a result of the wedding, she'll be "Marion John" (say it out loud), Garfield interprets the remark as the woman being Jon's secret fiancee and spends the episode worrying that Jon won't pay as much attention to him after the wedding, but more that their children will play harmful games with him. At the end, he discovers what the viewer knew from the beginning: that the woman is Jon's cousin and she is marrying a man whose last name is John. His attempt to disrupt the wedding is fortunately taken in good humor and the wedding continues.
- 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.
- The Simpsons:
(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.
- 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. To be fair, the guys probably thought that Bart and Lisa were only friends instead of siblings.
- 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 turns out to be his cousin—Jeremy reveals that she was actually talking about how she was trapped on a videogame level, and that it took her at least week to get out of it.
- 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.
- 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 Daria to date, and partially because they know Tom was recently involved with Daria's friend, Jane.
- 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, Fred asks if she's an ex-girlfriend. Pietro responds "worse - she's my sister".
- On 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 fiancee, Mordecai spends the episode in a depression. At the end we find out that the guy was her cousin, John, and the reason he was bending in front of her was because he was showing her his 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.
- In Codename: Kids Next Door, Nigel's girlfriend, Lizzie, discovers him with another girl. Thinking they're on a date she confronts him about it creating a huge scene. Nigel explains to her that the girl in question is his cousin, and instead of being relieved, she gets disgusted, still believing that it's a date.
- On Family Guy Brian is trying to build a friendship with Quagmire, who 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?!
Brian: I, uh, I notice you got a new receptionist. Nice little body on her, huh?
- He makes a similar mistake in "The Thin White Line" while making awkward small talk with his psychiatrist:
Dr. Kaplan: That's my daughter.
Brian: ... We could probably call this an early day, huh?
- Rocko's Modern Life. In "S.W.A.K." Rocko is smitten with his new mailcarrier, Sheila, 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 Sheila'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 Sheila, who notes that Wallace is her brother, much to his relief.
- Mayor Dewey believes the Gems are Steven Universe's older sisters. They're actually his guardians and Parental Substitutes.
- Jem And The Holograms: 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.
- In Voltron: Legendary Defender, Pidge has a picture that the others assume to be of him and his previously-unknown girlfriend. It's actually a picture of Pidge and her brother.
- 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 emphasising 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 pretty funny.