Warning: Spoilers Ahead.
Relatives getting mistaken for Love Interests in Literature.
- Sue Townsend's Adrian Mole, when aged fifteen, is allowed to witness the birth of his sister because the nurses assume he's actually the baby's father.
- In Bag of Bones by Stephen King, Mike learns that before his wife Jo's death, she had been seen hanging around TR-90 with a man and being openly affectionate with him. He naturally assumes that his wife has been having an affair until he confesses his fears to Jo's brother Frank, who reveals that he was the man with whom Jo had been seen.
- The Berenstain Bears: The chapter book The Berenstain Bears and the School Scandal Sheet has classmate Queenie McBear put an unapproved article in a secondary (and unauthorized) school newspaper about Teacher Bob being seen having dinner with a pretty teenager, causing a fuss and an inquiry for unprofessional behavior. Queenie's editor-in-chief Brother Bear angrily informs her that had she not gone over his head (which ultimately led to them being exposed as the writers of the secondary newspaper), he would have killed the story on the spot — he knew full well that the girl in Queenie's photo was Bob's teenage niece, who was in town to visit him.
- In David Eddings' The Belgariad, Ce'Nedra makes this mistake about Belgarion and his cousin. Her response to being corrected is, amusingly, equal parts "Whew, he's not attached" and "Shit, I'm in love with him!"
- In one of Kathy Reichs' Temperance Brennan novels, several other characters mention seeing her boyfriend Ryan going out with a young woman. It's eventually revealed that she is his recently discovered daughter.
- 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 .
- In The Dresden Files, this becomes a Running Gag:
- In Blood Rites, between Thomas Raith and Harry Dresden, made worse by the fact that 1) they cannot openly reveal their true relationship, and 2) they were sharing the latter's small apartment for an extended period after Blood Rites. And 3) Thomas was deliberately pretending to be gay in his public life for completely unrelated reasons. Considering all this together, we should have seen it coming from the paragraph after that Wizard of Oz poster on Thomas's wall.
- In Dead Beat, Waldo Butters (after meeting Thomas for the first time), is hurt that Harry hasn't previously come out to him. Thomas deliberately kisses Harry on the head and pours him tea after listening to Harry's subsequent startled denial.
- In White Night, this is deliberately exploited to explain to building security why Harry has a key to Thomas' apartment and has turned up there in the latter's absence.
- Roger Ebert listed a variant of this trope in his analytical book Eberts Glossary Of Movie Terms as the "Inevitable Sister".In any movie where the heroine catches her boyfriend dancing in public with another woman and makes a big scene, the other woman invariably turns out to be the boyfriend's sister. Cf. MYSTIC PIZZA, etc.
- Forest Kingdom: In the Hawk & Fisher spinoff series' Book 3 (The God Killer), when investigating the members of the God Squad, Fisher tracks one of them, Charles Buchan, to the base of the Sisters of Joy, who are basically a religion dedicated to sex and pleasure. She assumes he's there to partake of what they offer, until he admits the truth — the woman he loved had died giving birth to their illegitimate daughter Annette, who's grown up as one of the Sisters, and he only recently found out about her.
- Fox Demon Cultivation Manual: Inverted. Because Song Ci drank Rong Bai's blood, his own blood smells like Rong Bai's. This prompts Zhu Yishu and (briefly) Rong Bai himself to assume they're related. The misunderstanding is cleared up long before they start a relationship, though.
- The Heroes of Olympus: In the first book, Piper, who has a crush on Jason, briefly wonders if the person in the photograph he's looking at is an old girlfriend he's just starting to remember. The audience already knows, and Jason later clarifies, that the photo is of the sister he recently found out he had.
- A Hole in the Fence: Variant. Grisón becomes shocked when he finds out that his friend Raclot thinks Grison is hanging with Prune a bit too much, since Grisón has never regarded Raclot's girlfriend as more than a friend. Later, after finding out that they are siblings, Grisón and Prune make sure to tell their respective love interests why no romantic relationship will ever happen between them.
- Keeper of the Lost Cities: When she first meets Tam and Linh, Sophie wonders if they're boyfriend and girlfriend, since they're sitting close together and sharing a fruit. It is only once she sees their faces that she realizes they're brother and sister.
- Isaac Asimov's "Liar! (1941)": Herbie, a telepathic robot, tells Dr Calvin that the man she's attracted to isn't dating. In fact, the 'other woman' that she had seen was a first cousin, and not his girlfriend. Subverted, as he is planning on marrying her soon after solving the problem of telepathic robots.
- The Moving Finger: The protagonist and his sister move to a rural English village. When a round of anonymous letters is sent to people in the village, theirs accuses them of being an unmarried couple posing as siblings (it doesn't help that they don't look related, one taking after their father and the other after their mother).
- Nina Tanleven: Unusual variant in The Ghost in the Big Brass Bed - Nine goes to Phoebe Watson's home to feed her cat while Phoebe is in the hospital, and while she's upstairs, a man also unexpectedly enters the house. Nine avoids being seen by him but does hear his voice and, afraid that he's a burglar, manages to sneak out in time to meet up with her father, get home and call the police. The next day, while she and Chris are in the hospital visiting Phoebe, the "prowler" also turns up to visit and turns out to be Phoebe's cousin Byron, whom Phoebe didn't expect to arrive until that morning. Fortunately, Byron is somewhat amused by the misunderstanding after Nine explains herself, and accepts her apology; it helps that Phoebe keeps his picture on her dresser, which he was able to use as proof that he really did belong there when the police came by.
- North and South by Elizabeth Gaskell. Similarly, the heroine Margaret Hale's reputation is damaged in the hero's eyes after she is observed saying an intimate goodbye to a young man at a train station, and then lies about it. She cannot reveal that the young man was her brother as he is a wanted man.
- Averted in Northanger Abbey when the heroine sees her Love Interest with another woman on his arm and immediately comes to the correct conclusion that it's his sister, thereby depriving herself of a perfect opportunity for needless drama, as the narrator tells us.
- In one of Anne Perry's Late Victorian mysteries, a married man is seen with a pretty young woman who he obviously cares for very much. It turns out she is his daughter, his legitimate daughter, as his second marriage is bigamous and his four children by it are illegitimate. Ouch.
- Reign of the Seven Spellblades: Katie, who has a crush on Oliver, is somewhat put-out when he's affectionately kissed on the cheek by a female upperclassman. Oliver identifies her as his cousin Shannon, and reminds her that he'd previously told The Team how she and her brother took him in after his mother died.
- Rob Roy: Frank sees Diana riding with a strange man, and he assumes that man must be her lover. Later, Frank learns he was Diana's father, Sir Frederick Vernon.
- In The Schoolmouse by Dick King-Smith, Flora's boyfriend Boy mistakes her father Robin for her suitor and bites through his paw. In a case of poor communication leading to fights, Robin doesn't outright say that he's Flora's father when asked by Boy, simply saying that he's come to see Flora.
- The Secret History: When Richard first sees Charles and Camilla together, he assumes they are boyfriend and girlfriend. He later learns that they are twins. Even later he learns that he was still right the first time.
- Special Operations: In the first book, Loki is jealous of a British commando who parachutes into the area and who Frey clearly knows and cares about. He turns out to be her older half-brother (who was sent into the mission because of his family ties in the area).
- Tarzan: In Tarzan and the Leopard Men, part of the plot involves a girl (who is nameless throughout most of the book, only being identified as "Jessie" in the last pages) who's searching for a man named Jerry Jerome, but refuses to say why. "Old Timer" (real name Hiram), who finds her early on, becomes jealous of this "Jerry Jerome" when he first hears of him, thinking the girl and Jerry are in a relationship. It's not until the last chapters of the book that Jessie and Jerry (revealed as "Old Timer"'s friend "The Kid") reunite, and reveal in the process that Jerry is actually Jessie's long-missing brother.
- 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.)
- Trapped in a Dating Sim: The World of Otome Games is Tough for Mobs:
- After Marie's Heel–Face Turn into The Lancer of the series, a few characters overhear her calling Leon "Big Bro" and come to the conclusion that they've become lovers and he has a sister fetish. Since nobody knows that various characters were reincarnated into this world from modern-day Japan, they're unaware that Leon and Marie were literally brother and sister in their previous lives.
- Queen Mylene proposes a match between Princess Erica and Leon since they've started having weekly tea parties with Marie as an apparent escort, with the ulterior motive of trying to bind Leon closer to the royal family in a time of unrest. This puts Leon in a bind, since Erica is actually the reincarnation of his niece from his previous life (born to Marie after he died in Japan), and he just wants to get to know her platonically.
- In Walk Two Moons, after Phoebe's mother leaves on an unexpected trip, Phoebe and Sal track her down and catch her with a guy in his late teens, who she's kissing on the cheek. Phoebe, who saw the guy earlier and thought he might have been a deranged killer, is not happy. When her mother returns, she brings him along and explains that he's her son from a previous marriage who she recently was reunited with. Sal also notices that her teacher, Mr. Birkway, and Mrs. Cadaver, the woman she and her father live with, spend quite a lot of time together and wonders if the two are in love. It turns out that they're brother and sister.
- Warrior Cats: Fireheart and Sandstorm encounter a strange kittypet she-cat who starts fawning over Fireheart and licking him furiously. Sandstorm wasn't Fireheart's mate but had feelings for him. Sandstorm is unnerved by the she-cat's affection, however Fireheart quickly mentions that she's his sister, Princess.
- E. Nesbit's The Wouldbegoods: This trope is cheerfully lampshaded. The child protagonists are reluctantly trying to reunite their adult friend with his long-lost sweetheart even though they fear this will mean he has less time for them. So they are pleased, rather than scandalised, to find the girl embracing another man. But their relief is short-lived and the narrator reflects, "He might have known it was her brother, because in rotten grown-up books if a girl kisses a man in a shrubbery that is not the man you think she's in love with; it always turns out to be a brother."
