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
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
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Anime and Manga
- 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.
- 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.
- 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 (the one with Pierce Brosnan), 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 Mc Clure 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.
- 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.
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.
- 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.
- Friends: 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).
- 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 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.
- 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 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.
- 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. They have a very close relationship and mixed-sex duos who are married are more common than 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.
- 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.
- 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. The girl turned out to be his daughter.
- The "Kiss Cam" at any sporting event usually 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 text saying "My Sister" when the kiss cam focused on them.