Mistaken for Incest

Nelson: Get a room, you two!
Lisa: [flatly] We're brother and sister.
Milhouse: So are my parents! I think...

— The Simpsons

Two characters are assumed by the people around them to be related. Maybe it was part of a con to explain why their "sibling" is living with them. Maybe they look similar and people jumped to conclusions. Maybe there was some legal weirdness that means it's technically true.

Then they start kissing, and people panic.

This can be presented to the audience in multiple ways. Sometimes the audience knows from the start that the characters aren't related, even if no one else does. Sometimes the audience believes they're related, and are in the same boat as everyone else. There's also the possibility of actual relatives being mistaken for being in an incestuous relationship when no such relationship exists. Sometimes the characters themselves believe that they are related after an initial attraction, a subverted case of Surprise Incest.

The inversion of Relative Error. Compare and contrast Surprise Incest (when the characters in the relationship find out they are related). Might involve a Relatively Flimsy Excuse. The conjunction of Mistaken for Related and Mistaken for Romance.

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Examples:

Anime & Manga

• Castle Town Dandelion: Akane finds her brother, Haruka, looking at pictures of her on a computer screen. She immediately thinks he's having those kinds of thoughts about her. She tells him, somewhat tellingly, that she's not opposed to the idea, just concerned about what their family and society would say. Turns out that he was just flagging images on a fan site for deletion because he knows how embarrassed Akane gets from public exposure.
• Heavenly Delusion: The hotel lady tells Kiruko and Maru that even in After the End they shouldn't get too close to each other because of all sorts of biological issues. Annoyed Kiruko tells her that they are neither siblings nor dating.

Comic Books

• The Cartoon History of the Universe: Then-contemporary gossip suggested that the ancient Athenian politician Kimon wouldn't let his sister Elpinike marry because he had an incestuous desire for (and possibly affair with) her. In reality, it was because their father Miltiades died owing the Athenian government a large fine, leaving his family rich in land but little else; there was no dowry for Elpinike, and since a dowry was a necessity for marriage in Ancient Athens, she couldn't get married until a wealthy man agreed to pay off the debt in exchange for her hand in marriage. This is punctuated by a two-panel sequence of Kimon telling Elpinike "I need you, sister... I need you to get me money."

Fan Works

• Naru-Hina Chronicles Mini-sodes: Hanabi is granted permission to call Naruto (who is dating Hanabi's sister Hinata) her big brother. As it turns out, she asked for it just to pull a prank: When Naruto is kissing Hinata, Hanabi takes this opportunity to yell "EWWW! MY SISTER IS SMOOCHING MY BROTHER!" Several bystanders hear what she just yelled and fall for it. This results in Hinata fainting once again, Naruto being angry at Hanabi, and the latter running away while laughing.

• Queens of Mewni: After one night where Venus got drugged and raped, she discovers all the nude paintings of her painted by her brother were stolen and distributed throughout the kingdom, leading to accusations of Brother–Sister Incest. That's one line Venus won't cross (and in fact banished her cousin from the kingdom for committing), but her reputation as 'Queen Moth' (moth in this case being a euphemism for whore) was already so prevalent that people were ready to believe she committed it.
• White Sheep (RWBY):
  • Qrow mistakenly believes that Jaune is his sister Raven's son, and therefore goes to a lot of effort to keep him from dating Raven's daughter Yang (who she abandoned as a baby). Making this also a case of subverted Surprise Incest, he's wrong.
  • Qrow does this to himself when he has sex with a girl who he then discovers is Jaune's sister, while he still believes Jaune is Raven's son. There is a lot of screaming.

Films — Live-Action

• Dasepo Sonyo: Poor Girl is briefly deterred from pursuing her crush Anthony after her mother mistakes Anthony for a son that she gave up for adoption years ago, as the son was adopted by a Swiss couple, just as Anthony was. It later turns out that Anthony is not her son; her actual long-lost son is a vagrant named Hans.
• Ferris Bueller's Day Off: Ferris poses as his girlfriend's father (distantly, wearing a disguise) in order to get her out of school. Then they make out in front of the principal, to his disgust.
  Rooney: So that's how it works in their family...
• Gone Girl: During the media circus surrounding the disappearance and apparent murder of Nick's wife Amy, speculation that Nick is having an incestuous relationship with his twin sister Margo is used to smear them both after they get implicated in what happened to Amy.
• Guardians of the Galaxy Vol. 3: The opening of the film shows that Peter Quill has become a drunken wreck while still mourning the loss of his girlfriend Gamora. Rocket suggests that Mantis, Peter's half-sister, should just "touch him and make him feel good". While Rocket was referring to Mantis using her empathic powers to make Peter feel better, Groot seems to think he was suggesting Mantis actually have sex with Peter, which grosses everyone else out.
• Nanny McPhee: The Brown children trick their evil great-aunt Adelaide into thinking the maid, Evangeline, is their sister because she wants to adopt one of them. Later, Mr. Brown and Evangeline admit that they're in love, and Adelaide, still thinking they're father and daughter, yells, "Incest!"
• Say It Isn't So: The entire plot involves two lovers whose relationship comes to an abrupt halt when a private detective tells them that they're long-lost siblings. When the guy finds out that the detective was mistaken, he sets off on a quest to find the girl, who has since moved to a different town.
• Shock Corridor: This is a big plot point, where the protagonist is an ambitious journalist that pretends to have an incestuous fixation on his own sister (she is actually his girlfriend) in order to be committed to an asylum where he is pursuing a story. Completely Played for Drama.
• Tommy Boy: Beverly and Paul are a married couple posing as mother and son in their con of "Big" Tom Callahan. After Tom dies at the wedding reception, they quickly move to sell the company to auto parts magnate Ray Zalinsky. Fortunately, Michelle catches Paul French-kissing and groping Beverly at the airport and she calls her brother, a policeman. Michelle then quickly informs Tommy of their deception who then exposes them during the sale thus saving the company, sending Paul to jail, and implying that Beverly would start dating Zalinsky.
• We're the Millers: At one point in the journey of drug dealer David and his three associates Rose, Casey and Kenny, who are pretending to be his family, Kenny develops a crush on their new neighbours' daughter Melissa, but awkwardly admits that he's never actually kissed a girl or had sex before. Casey, taking pity on him, offers to give her "brother" a Practice Kiss, whereupon David and Rose walk in on them doing it. Since David knows neither of them is related, he's okay with it, and Rose then offers to teach Kenny a special trick he can do with his tongue to liven up the kissing... and then Melissa walks in on them, and since she doesn't know none of these people are actually family, she thinks Kenny is kissing his actual sister and mother, and it doesn't end well.

Literature

• A Feast for Crows: Cersei Lannister observes her much-loathed daughter-in-law Margaery Tyrell and how well she gets along with her brother Ser Loras. She starts wondering whether they are engaging in an illicit relationship. It is hinted that it is just Psychological Projection on Cersei's part, given her own Twincest with her brother Jaime.
• Gone Girl: As male-female twins who were one another's lifelong best friends, Nick and Margo had to contend with rumors of Brother–Sister Incest in high school; Nick makes a semi-humorous point of stating for the record that he and his sister have never had sex, or thought about having sex, or done anything other than like each other a lot in a completely sibling-appropriate way.
• The Hound of the Baskervilles: The Stapletons are deliberately posing as brother and sister when they are actually husband and wife, one of Stapleton's farsighted ideas as it permitted him to seduce Laura Lyons as part of his plot to murder Sir Charles, and to get an infatuated Sir Henry to more easily accept his invitation to dine with them. However, he nearly betrays himself when he acts the Crazy Jealous Guy on seeing Sir Henry trying to woo his wife. Holmes finding out about their relationship is the point where Stapleton is pegged as the Big Bad.
• Reincarnated Mage With Inferior Eyes: Abel and Lilith are in a relationship, while they are pretending to be brother and sister to explain why they live together. (In truth, Abel recently reincarnated himself from 200 years ago, while Lilith is his long-lived demon confidant.) Eliza, who has an unrequited crush on Abel, catches them kissing early on and has to be convinced by a third party that it was only platonic affection.
• Yumi and the Nightmare Painter: Yumi introduces herself to Akane as Painter's sister... and then mentions how they took a bath together recently. It might be for the best that all that got buried under Yumi mistaking Akane for Painter's concubine. At the end of the story, when Yumi is saved, Painter has to explain that the girl he's suddenly kissing a lot is not his sister, but his "girlfriend from another city." Apparently, his friends still didn't believe it until his parents showed up and confirmed they didn't have a daughter.

Live-Action TV

• Friends:
  • In an early episode, Joey lies to Monica that his ex-girlfriend and her new boyfriend are siblings to trick her into a double date with them in an effort to win back said ex. Monica soon becomes repulsed when the "siblings" start acting inappropriately, leading her to believe Joey set her up with a sibling couple. She eventually realizes he lied to her, however.
  • Later when Phoebe announces that her half-brother and his wife asked her to carry their child, Joey, who knows nothing about surrogacy, initially assumed Phoebe was going to have sex with her brother. She's rightly disgusted.
• How I Met Your Mother: In a flashback, Barney Stinson is disgusted to see his ex-girlfriend kissing a guy he thought was her father. However, it turns out the guy wasn't actually her father at all.
• Just Shoot Me!: In "Where's Poppa?", Jack suspects Maya's new boyfriend is his own son (having had a one-night stand with the young man's mother about the time he was conceived) and thus Maya's half-brother. Nina stops anything physical from happening, but the mother reveals that he isn't Jack's. He's so upset about the insensitive way Maya explains the situation (his actual father having just died and all) that he breaks up with her anyway.
• Picket Fences: In one episode, Kimberly Brock sees her best friend kissing her father. She reports it to her own father, the town sheriff. In court, it's revealed by Douglas Waumbaugh that the young woman is not the man's daughter. The man is a polygamist, and the girl is his second wife, posing as his daughter publicly, with her "mother" being the first wife. Judge Bone points out that the laws against polygamy may one day be challenged on religious grounds to the Supreme Court, and he doesn't want Waumbaugh to be the one who brings the case, so urges the town to get the man to dissolve one of his marriages, preferably his 18-year-old bride, the one that started the whole mess in the first place.
• Veep: While Selina is juggling secret negotiations with the Chinese and a family Christmas party, Minna Häkkinen and the Chinese delegation spot Marjorie — a former Secret Service agent hired for her resemblance to Selina — kissing Selina's daughter Catherine. Unsurprisingly, they believe they saw Selina making out with her own daughter. Minna sternly lectures Selina about it later, but Selina thinks she's talking about an unrelated event with Catherine and just laughs it off as a mild faux pas. Neither Minna nor the Chinese President is pleased.
• Veronica Mars: This was Season 1's explanation for Veronica's rape. She and Duncan had consensual sex under the influence of GHB. Duncan had previously broken up with Veronica because he mistakenly thought that Veronica was his sister, as his father and her mother had had a long-term affair. Ultimately, Veronica learned that they are not biologically related.
• The Vicar of Dibley: In "Love and Marriage", Geraldine's enjoyment of Alice's hen party is disrupted when Alice lets slip that her Disappeared Dad was "David Horton"—the same name as her fiancé Hugo's father. She's much relieved to learn from Owen the next day that Alice meant an entirely different member of the Horton family; Alice and Hugo are second cousins, not half-siblings, and everybody else in the village has known that for years.

Video Games

• Fire Emblem: The Sacred Stones: The Brother–Sister Team Ephraim and Eirika are frequently interpreted as having romantic feelings for each other by fans, because of a combination of it not really being all that out there for a series that's been known for all sorts of incest for ages, and some subtext that no one can agree is intentional. What makes this a definite example is that this belief was referenced by Fire Emblem Heroes, with Ephraim referring to it as a disgusting lie. The word "disgusting" then became a meme..

Web Animation

• Helluva Boss: In "Unhappy Campers", married couple Moxxie and Millie infiltrate a summer camp posing as teenage siblings named Moxxine and Millerd. Near the end, after "Millerd" has become a social media celebrity and the two make up after a fight, they start having hot sex in front of a horrified crowd, causing "him" to gain a deviant reputation and lose "his" cred.
  Newscaster: Internet sensation and local celebrity Millerd Realboy caught incesting tonight, showcasing a sickening display onlookers deemed "Too disturbing to look away, and we won't."
• HFIL makes a gag of this twice, both times involving Cell:
  • When he first wakes up in HFIL and meets the pair of Goz and Mez, whose appearances and similarities make their relationship ambiguous, Cell says that they're "charming brothers, or lovers, or both, I don't judge..."
  • When Freeza's brother Cooler visits HFIL and Cell is wondering why Freeza is so antagonistic toward him, he guesses that Cooler was his ex-lover. Freeza is very quick and loud to correct him that he's his brother. Cell however is still under the impression that Cooler is Freeza's ex since he didn't correct him about that.
    "Again, I don't judge."

Web Videos

• Wayward Guide for the Untrained Eye: A Running Gag sees fraternal twins Artemis and Paul mistaken for a married couple by the Connor Creek townsfolk. When they correct them, saying they're siblings, the townsfolk are disgusted.

Western Animation

• American Dad!: In "Pulling Double Booty", Francine walks in on Hayley kissing her father Stan's stunt double Bill. Francine faints at the sight of this and angrily accuses "Stan" of making out with his daughter. Even when Stan comes to explain to her that Bill is just his body double, she can't help but be disgusted.
• Family Guy: In "Screams of Silence: The Story of Brenda Q", Brenda is overjoyed that her brother Quagmire has awakened after he's nearly died from Erotic Asphyxiation, and she hugs him in relief. Her boyfriend Jeff becomes pissed and yells at Quagmire to get away from his lady. Considering this is Quagmire, an acknowledged rapist who once implied he'd date his infant daughter once she turned 18, this might be the only instance where Jeff had a point.
  Peter: Relax, Jeff, that's her brother.
  Jeff: Yeah, that's how it starts!
• Tiny Toon Adventures: Avoiding problems like this is presumably why Babs and Buster Bunny chime in with their "no relation" catchphrase when introducing themselves.
• Young Justice: M'gann and Conner's first kiss happens while they are on an undercover mission in Belle Reve Penitentiary as Tommy and Tupence, the villainous Terror Twins.
  Icicle Jr.: [disgusted] Dude, that's your sister!

Real Life

• Donna Tartt wrote a letter to her friend Jonathan Lethem in 1983, recounting an anecdote about her and her at-the-time boyfriend Paul McGloin getting mistaken for an incestuous couple. This is theorized to be the origin of the idea that later became the incestuous twins in her novel The Secret History.
  The letter: Paul & I were almost kicked out of our lodgings last Tuesday. The charges? Incest. That's really rather impressive, isn't it? Fortunately we are not brother and sister, or else we would have been quite guilty.

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