Adultery is considered to be a betrayal on the cheater's behalf, and a major source of drama because of it. And as family is supposed to look out for one another, when the person they cheat with is a relative of the cheated partner it's all the more of a betrayal, and thus all the more dramatic. Usually it's the wife sleeping with her brother-in-law, or brother sleeping with his sister in-law, though sometimes it's the parent or child. Doing so means they're almost universally considered a Jerkass for betraying their own loved one's trust, and the affair is usually on the bad end of Good Adultery, Bad Adultery. Of course, it's possible for it to be sympathetic if the relative is far nicer than the spouse being cheated on, or the cheaters wanted to be partners in the first place but had to settle for the other relative. Maybe it's just a sign of a Dysfunctional Family.
While it can involve Settle for Sibling and Love Father, Love Son, those tropes are usually for when the partner is dead or they're not in a relationship, if they ended up pursuing it at all. Of course, it's possible that it leads to a Remarried to the Mistress scenario. The cheating partner might find their in-law to be a Replacement Goldfish to whatever spark they had with their spouse. If the relative turns out to be the father of the spouse's child, it's a Family Relationship Switcheroo. Usually doesn't have anything to do with Incest Is Relative although some cultures may consider it semi-incestuous, unless the partners were already related.
- Berserk: The Queen of Midland was the king's second wife; feeling neglected by him, she ended up carrying an affair with his brother Count Julius. While the relationship was purely physical, after his death she came to realize she did truly love him.
- Kanon by Chiho Saito: Gen Mikami sleeps with his fiancee Maria's mother. Maria doesn't take it well.
- Revolutionary Girl Utena: Akio ends up sleeping with his fiancee's mother. He's a lot more of a Manipulative Bastard about it, though.
- Batman: Commissioner Gordon was originally meant to be Barbara Gordon's uncle before being retconned to have always been her father. Barbara once discovered that he may have been sleeping with his brother's wife around the time she was conceived, implying she may have been his biological daughter.
- Spider-Man: Norman Osborn had an affair with his son's fiancée Lily Hollister. This isn't even the worst thing he's done to Harry.
- The HBO movie Above Suspicion, starring Christopher Reeve and Kim Cattrall as husband and wife Dempsey and Gail, in which Gail is having an affair with Dempsey's brother Nick. Dempsey ends up killing them both and successfully spinning it as self-defense.
- Hellraiser: Judgement: Sean's brother David, who is also his colleague and partner in the police department, is having an affair with Sean's wife Alison. Sean tries to use this sin to offer them as a sacrifice to Pinhead in exchange for his own soul.
- Ken Park: The teenager Shawn is sleeping with his girlfriend's mother behind her back and fantasizes about having a threeway with the two of them. Sadly, his situation is probably the least psychologically messed up of all the four main characters, whose lives are rife with abuse and dysfunction.
- Very early in Kiss Me Guido, protagonist Frankie walks in on his fiancee having sex with his brother on the kitchen counter. This betrayal launches the plot, as it drives Frankie to move away and pursue his acting dreams.
- In Love Actually, Jamie's storyline begins with him discovering his girlfriend is having an affair with his brother.
- In Soul Food, Nick cheats on his wife Teri with her cousin Faith.
- In How to Make an American Quilt, it's eventually revealed that the long-standing issue between sisters Hyacinth and Gladiola is that while Hy's husband was dying in the hospital, Hy slept with Gladdy's husband Arthur.
- In Bumble Bee, Roy, the corrupt hick who explodes into goop by Decepticons Shatter & Dropkick when they first arrive, apparently attempted (and failed) to cheat on his wife with her sister, and cares more about his car than he does the lives of anyone around him.
- An old joke:
''A man goes to meet his fiance's family, and while they all seem friendly enough, he notices that her younger sister seems quite smitten with him, contriving to jiggle her cleavage whenever he looks in her direction, dropping things and bending over to pick them up while next to him, playing footsies under the table at dinner, etc. Finally he ends up alone in a room, when the sister enters half-naked and orders him in no uncertain terms to ravish her then and there, adding that her sister will never know anything about it. Without a word, the man gets up, leaves the room, goes down the stairs, exits the house, and already has his car keys out when the rest of the family pops out of hiding, the father putting down his baseball bat to congratulate him on passing the faithfulness test.The moral of the story: Always keep your condoms in the car.''
- Cloud Atlas: The main reason why Timothy Cavendish is locked up in a home is because he slept with his brother Denholme's wife Georgette. They seemed to have a bad relationship before this, but it certainly didn't help.
- In East of Eden, Catherine cheats on Adam Trask with his brother Charles.
- In The Folk of the Air, the heroine Jude Duarte starts a relationship with Locke. She later discovers that he is in a secret relationship with her sister Taryn, whom he even later marries.
- In Hurog, Ward's father in the backstory had an affair with the half-sister of his wife (Ward's mother).
- Judge Dee: One case involved a young man having an affair with one of his father's young concubines, even planning to murder his father and frame a local artist who claimed the father was a traitor to his country. Unfortunately, in Confucianist China the issue of disrespecting one's father is so important that the son might as well have been screwing his biological mother, and the judge indirectly orders him to commit suicide (the father was murdered, but for entirely different reasons by someone else).
- Modern Faerie Tales: In the novel Valiant, the teenaged heroine Valerie Russel discovers that her boyfriend Tom is sleeping with her mother. This causes her to run away from home due to feeling betrayed.
- A Song of Ice and Fire
- The justification for the Blackfyre Rebellion's cause is the rumors that the current king, Daeron Targaryen, wasn't actually King Aegon IV's son. Rather, he was the result of Queen Naerys cheating on his, or rather their brother and kingsguard Aemon the Dragonknight. Since Dameon Blackfyre was a legitimized bastard of Aemon and Daena the Defiant, he and his descendants would have more of a claim to the throne if it were true. In-universe most believe this is unlikely due to how pious and honorable Naerys and Aemon were, even if they did love each other, and that Aegon himself started the rumor in order to disinherit his son.
- After her brother/lover Jaime (with whom she was sleeping throughout her marriage to King Robert) is captured by the Starks, Cersei consoles herself by sleeping with their cousin Lancel, who is apparently closely enough related that he resembles Jaime.
- "Black" Walder Frey is rumored to have slept with his brothers' and cousins' wives along with his female cousins, and thought to have slept with his great-grandfather Walder Frey's young seventh wife Annara Fanning. This puts the paternity of the younger Freys into question, making House Frey even more of a Big, Screwed-Up Family than it already is.
- The root cause of Victarion and Euron Greyjoy's feud is that Euron slept with his wife, which Ironborn culture considered her Defiled Forever because of that and Victarion killed his wife out of "honor". While he suspects he raped her, Euron claims she was "wet and willing" and the betrayal is the same. Sadly, this isn't even the worst of what Euron has done to his family or people in general.
- This happens a lot in All My Children:
- Maria cheated on her husband Edmund with his brother Dimitri. She soon after got pregnant, and it wasn't clear who the father was. A Daddy DNA Test revealed Edmund to be the father.
- Gloria cheated on her fiance Stuart with his twin brother Adam. Stuart broke up with Gloria once he found out, and Gloria and Adam ended up getting married ... with Stuart as the best man.
- Tad cheated on his girlfriend Liza with her mother Marian.
- Arlene blackmailed her daughter Hayley's fiance/husband Alec into having an affair with her.
- In the 2000 miniseries Arabian Nights, Sultan Shahryar's distrust of women has its origin in the discovery that his wife was carrying on an affair with his brother.
- Doctor Who: Captain Archibald Hamish Lethbridge-Stewart from "Twice Upon a Time" was intended by Steven Moffat to be the Brigadier's grandfather, however according to Henry Lincoln and the Haisman estate (who own the character and publish the spin-off Lethbridge-Stewart series), he was supposed to be his great-uncle, and Alistair Lethbridge-Stewart was the grandfather. A compromise was made with a short story that revealed in a moment of loneliness while Alistair was off on a mission Archibald and Alistair's wife Lillian had a secret tryst, strongly implying that he was the Brigadier's biological grandfather, and Alistair his legal grandfather.
- One episode of CSI: Miami has this as its big reveal. The victims in a double murder are the parents of a teenage girl. The girl's boyfriend killed her father, and the girl arrived in time to see her mother telling the boyfriend that because of what he did, "We can be together now" - he had been sleeping with the mother. Enraged, the girl killed her mother. When the shock wore off, the two teenagers agreed to keep silent.
- On The Get Down, Lydia had quite a bit of sexual tension with her brother-in-law Francisco, who clearly cares more about her and her daughter Mylene than her husband Ramon. It's implied something may have happened between them in the past, which eventually gets confirmed when she reveals to Ramon that Francisco is Mylene's (and possibly her younger daughter's) actual father.
- Voltaire: In "Stuck With You," a husband and wife are both complaining about the other, with increasingly escalating problems. One verse shows that the couple had cheated on each other, starting with the husband:
Wife: You promised you'd be true to me,
But gave me infidelity!
Husband: Why should I give a tinker's cuss?
She was your sister, what's the fuss?
- Classical Mythology:
- One of the origins of Zagreus and Melinoe is that Zeus slept with his brother Hades's wife Persephone by disguising himself as Hades. Persephone also happened to be Zeus's daughter by his sister Demeter.
- Ares's favorite consort is Aphrodite, who was married to Hephaestus, blacksmith god and his full brother. While Aphrodite cheated on her Unwanted Spouse with a lot of other people, Ares was the one best known for it.
- Book of Exodus:
- Several paragraphs are dedicated to admonishing the Israelites not to have sex with family members, including stepparents/stepsiblings/in-laws/etc. Doing so was punishable by the parties involved being burned alive.
- Later, in one of the prophetic books, after Israel has turned (yet again) to idolatry and all-around depravity, the writer laments that a father and son visit the same temple prostitute, or "use the same girl." (Maybe at different times, maybe at the same time; the text doesn't say.) The writer considers this to be a form of incest.
- Chicago: Velma, like all the other "Merry Murderesses" of the Cook County Jail,is a murderer; she killed her own husband and twin sister. In "Cell Block Tango", her motives were explained to have been blind fury at finding her husband and sister having sex after she left for just a few minutes.
- In Hamilton, Angelica remains in love with Alexander even after he marries her sister, and he holds some longing for her as well. This is a reference to the theory some historians believe that the real Hamilton may have had an affair with sister-in-law Angelica.
- Hamlet: After poisoning his brother, King Claudius is quick to comfort the widow and marry Queen Gertrude. It's implied in some adaptations that the two were carrying on an affair while the king was still alive.
- In The Lion in Winter, the French princess Alys has been betrothed to Richard the Lionheart since she was quite young, and has been raised in the English court to prepare her for that role. However, for some time before the start of the play, she's been the mistress of Richard's father, Henry the Second. Henry's wife, Eleanor of Aquitaine, seems to take it fairly in stride and even informs Henry that early in their marriage, she cheated on him - with his father. Henry does not take this in stride, and is in fact physically ill at the idea. (It's hinted that she might be lying just to upset him.)
- Metal Gear:
- Legally Meryl Silverburgh is the son of Roy Campbell's brother Matt, who was killed in the Gulf War. Eventually, it's revealed that he's her real father, as a result of an affair he had with Meryl's mother/his sister-in-law.
- The wife of Huey Emmerich at one point had an affair with his son (her stepson), Hal. This led Huey to commit suicide and his wife Julie eventually remarried. Her new husband later tried to sexually assault her daughter and was seriously injured in self-defense.
- In The Walking Dead Season 3, Javier has the option of reciprocating the feelings of his sister-in-law, Kate, whose husband and Javier's brother, David, is presumed dead. Even before the outbreak, Kate and David's relationship was already strained due to him being unpleasant. He's even confided to Javi that he's realized he hates being a family man to the point that he wants to leave and entrust his wife and family to Javi. When it turns out David is alive, it complicates things. He'll be upset when he thinks Javi and Kate are together and accuses Javi of stealing his family (regardless of whether or not Javi has been accepting Kate's advances, Kate deliberately pushes David's Berserk Button at one point by suddenly kissing Javi). In the ending, Kate won't feel bad if David was killed and urges Javi to do the same, though if Gabe was also killed, she'll ask for some space, but will otherwise want to pursue their relationship.