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. Compare Mistaken for Related. If the two siblings are the same gender, then this also crosses over with Mistaken for Gay.
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.
- In one Blondie (1930) 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!
- 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 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.
- In The Train at Platform 4 episode "Gilbert", Gilbert tells the train manager, Sam, that he's been lying to his family about having a more upmarket job than first class rail steward, and he's just seen his stepsister get on the train. (In first class, naturally.) Sam agrees he can swap places with Dev in the buffet car, but rather than explain the whole thing to Dev, she just tells him there's a passenger Gilbert doesn't want to deal with. Dev responds with a knowing "Right. It's a woman, yeah?", so Sam has to explain it anyway, leaving Dev briefly believing Gilbert had a one-night stand with his stepsister.
- 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 tries 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.
- Tales of Zestiria: In one of the Earthen Historia, the party witness a flashback to the author of the Celestial Record founding a village, alongside a woman holding a baby. In the English localizationnote the Earthen Historia linked to this event describes the woman as being his wife, and the baby their son. The woman is actually his sister, Muse, and the child is his nephew, the human Mikleo respectively.
- An ironic one comes up early on in Daughter for Dessert. Moe Mortelli, on seeing Amanda, thinks she's 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 revealed in the epilogue of the 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.
- Not Always Right and its spinoffs:
- Inverted in one story; 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.
- In this Not Always Working story, a sacked cleaner tries to blackmail the owner by telling his wife he's been seeing another woman. The owner is furious ... because he and his sister have been trying to arrange a surprise party and now it's ruined. As the owner, his wife, and the submitter (the replacement cleaner) go back inside, the sacked cleaner remains standing there, apparently still trying to process what just happened.
- Pirates SMP: On Day 2, Scott starts to ask Owen if Acho, whom he'd been talking to and questing with earlier that day, has been saying anything about stars past. Owen immediately gets suspicious of the questions… and assumes that Scott's trying to get a date. Scott is, in fact, Acho's older brother who's been concerned about star after star left home without notice a year before the start of the SMP, and whom he thought was dead until the previous day. The conversation then immediately takes an awkward turn.
- 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.