The Game The Whole Family Can Play!Haha! Guess what you were doing?
Sometimes in stories, two people meet and just seem destined for romance. Love blossoms in a particular way, or perhaps the two have a loveless Arranged Marriage lying in wait for them instead. But then, someone makes a startling discovery — genetics has struck! The couple-to-be are related — and not in the "same species" sense — and the romance or arranged marriage is called off. The result may be either great rejoicing or great despair. If it's not called off despite the consanguinity issue, intentional Brother-Sister Incest results. Or if the two still want to be together and the author doesn't want to go the incest route, a counter-reveal may show up to prove that the genetics aren't a problem after all.
These days, many people are squicked out by this, so the characters often find out before they tie the knot/do the nasty. This wasn't always the case; Oedipus was very happily (but unknowingly) married to his own mother for many years and had several kids with her before things fell apart (to the extent that The Reveal resulted in Oedipus's mother killing herself and Oedipus himself stabbing his own eyes out), so this is Older Than Feudalism. Still, this trope is surprisingly common in anime and manga, as well as other Japanese entertainment, due to much more relaxed views on the subject in Japan.
The surprise relationship is usually either brother/sister, half-siblings, parent/child (much rarer these days than it was in ancient Greece), or, very rarely, uncle/niece or aunt/nephew. Cousins are only borderline squicky since marriage between cousins is actually legal in some regionsnote "Some regions" being almost every country on Earth and roughly half of the United States, and is quite acceptable in many romance novels. In fact, in the Arabian Nights, a man will call a woman he loves or is married to "daughter of my uncle", because in that time marriage between cousins was considered the best option! Because of this, it almost never appears as a plot point since it's a little too close for some folks to be comfortable, but not close enough that the plot demands the relationship break up.
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Anime & Manga
Onegai Twins played with this one. Each girl came believing that she was the twin sister. They find out about each other, and realize that they each have a 50/50 chance of being the twin. They both fall in love with the main a few episodes after this. The irony being the anime had one of the girls be the guy's sister, and the novelization had the other one.
The manga Maya's Funeral Procession has the main character, Reina, falling in love with a mysterious girl named Maya, only to discover that Reina is the product of her mother's affair with Maya's father and thus they're actually half-siblings.
Koi Kaze has 27-year-old Koshiro and 15-year-old Nanoka having a chance encounter, going to an amusement park together, forming an emotional connection and spark of mutual attraction, and then finding out they're brother and sister in the first episode. This turns into a deconstruction of Brother-Sister Incest, as the two go on to develop a painful and guilt-ridden romantic relationship against their better judgment, fully aware of the devastating implications if their secret got out, but both unwilling to live without the other.
In .hack//Legend of the Twilight, Shugo and Rena are a pair of brother-and-sister twins who are separated offline in real life due to their parents' divorce ( they each live with their same-sex parent ). They are reunited online in "The World", the MMORPG where the .Hack// franchise takes place, where they become the series' main Official Couple in a Relationship Upgrade. In the original Manga version, they were simply siblings, even showing Rena convincing Shugo to play the game with her, and they never went beyond good siblings. This also probably explains why that anime was quickly declared Canon Discontinuity by Word of God, with the manga becoming the canon.
Depending on your interpretation of their relationship before the big reveal, Kira and Cagalli of Mobile Suit Gundam SEED. They never really got around to doing anything, but they definitely looked like the usual romantic pairing all the way up to the last five episodes of the series. Especially since the first half of Gundam SEED was heavily based on the original Mobile Suit Gundam, with Kira and Cagalli having roles roughly analogous to Amuro and Sayla. Word of God is that this was a deliberate reference to Luke and Leia.
Negi and Asuna, one of the most popular and teased pairings in Mahou Sensei Negima!, are very distantly related by the Lifemaker, maybe closer, but how close has yet to be revealed..
In the Cain Saga, unbeknownst to him the main character's mother is his father's older sister. He then proceeds to fall in love with his first cousin, which was legal in the time period. The problem is, his cousin is his father's older sister's daughter, making her also his half-sister. There are a couple other brother/sister couples as throwaway characters, too.
Averted in Slayers: Thankfully, Zelgadis stating that Rezo was both his grandfather and great-grandfather turned out to be a translation error.'
Falan and Ilsaid's dad in Vampire Game did not inform them that they were related, until his son asked for his sister's hand, that is. He then only told his Un Favorite, though.
Detective Conan has a case with Raita Banba and his fiancee, Hatsune Kamon. Uniquely, it turns out that they're twins and that the latter has Turner syndrome. Hatsune suspected something was up upon learning that in addition to their identical birth dates, blood types, and unusual ability to know what the other was thinking even when they were silent, that they'd both been adopted after being orphaned in the same fire as infants. When she gets the test results confirming it, she... doesn't take it well. And for worse, Raita is accused of having killed her in a Yandere fit. Conan, through Kogoro, has to clear his name.
Vampire Knight: when Kaname turns Yuuki into a vampire, restoring her memory, she reveals to the audience that Kaname is her brother. Ah, so the Love Triangle will be easily solved, won't it? Wrong. Kaname confirms she's his sister, and states she was born to become his wife. Then he asks the other why they look surprised — this is very common among pureblood vampires, and in fact their own parents (Juri and Haruka) were siblings and Happily Married.
The prince and princess from Andromeda Stories, a pair of Half-Identical Twins who were Separated at Birth, get into a relationship before learning of their heritage. Weirdly, they decide to stay together after the truth comes out, citing the fact that incest is fairly common in royal families.
Kanba and Masako subvert this in Mawaru-Penguindrum. They know about their bond and have it more or less clear ( though Masako has quite the case of Big Brother Attraction as shown when she kisses Kanba), but it's a surprise to the audience and to Kanba's adoptive sister Himari. Doubles as Twincest.
In Daily Lives of High School Boys, Emi, after days of Cannot Spit It Out, finally finds the chance to confess her love to the visiting Hidenori on a festival, which coincides with his last day in town which happened to be his maternal hometown. But just when she thinks she finally found her chance, Hidenori makes a most surprising revelation to her: they are cousins.
Emi:"And so, I ended my summer break without being able to do a thing."
A possible example is Millernium "Milly" Feria Noctum and Kane Blueriver in Lost Universe, although, since they are second-cousins, they're probably distant enough to date if they really wanted to, and the very open ending lends serious doubts as to whether Kane survives the end of the series anyway.
In Part 6 of JoJo's Bizarre Adventure, a baby switch eventually led to Pearla Pucci dating her older brother, Wes (aka Weather Report). Enrico, upon discovering the relation, tried to keep the damage to a minimum by not telling her and instead hiring a third party to make them break up. Too bad the third party's method of doing this was to lynch Wes...
In Saiyuki, fraternal twins Cho Gonou and Cho Kanan were separated at a very young age when their parents divorced, one parent taking each child. Years later, they met at school, fell in love, discovered their relationship to one another, and became lovers, in that order.
Jack from Jack of Fables doesn't find out that he's adopted and that the Page sisters are also his half-sisters until after he's already slept with each of them. He takes the news relatively well, his sisters, not so much.
It may never have been revealed to the characters, but Cutter once got it on with Kahvi during a pre-battle orgy, and Kahvi takes up with him while Cutter's family is lost in time, though it doesn't really work out. Much later, readers learned that Kahvi is the daughter of former Wolfriders chief Two-Spear. Cutter is a direct descendant of Two-Spear's half-sister, Huntress Skyfire, making Kahvi his cousin four times removed. Which, especially considering the small gene pool of the Wolfriders, isn't spectacularly close. And with Recognition preventing conception for any but good genetic combinations, the elves may not have an incest taboo.
Also, if one looks at Scouter/Tyleet's relationship, you'll realize they're 2nd cousins (Brothers Longbranch and One-Eye, Longbranch sires Nightfall, One-Eye sires Scouter, Nightfall has Tyleet, Scouter and Tyleet Recognize). It makes sense, however, that such in-pairings will happen in such a small tribe- so yes, Recognition probably does take care of the biggest problem with incest. Thankfully, however, we haven't been subjected to any closer pairings...
A story arc in The Fabulous Furry Freak Brothers has Fat Freddy looking for his old home in Cleveland, broke, hungry, and bemoaning his perpetual bad luck. Just then, to his delight, a cute girl appears, befriends him, takes him home, feeds him, and joins him in the bathtub. A car pulls in and she warns "Here come my parents!" - Freddy sees them and exclaims "Holy Toledo, these are my parents too!"
In a list of cliche movie props, one Arab beggar reunites his necklace with that of the Caliph's daughter, whom he loves, and in doing so, learns that they are brother and sister. The love affair ends before it starts.
The doujinshi Golden Sun Gag Battle! featured a 4koma in which Ivan's first use of Reveal was to spy on Hama through her clothes. Gag Battle was based on the first game and published prior to the release of the second, which revealed that Hama was Ivan's sister, making this a meta example.
Not to mention the fandom's unofficial name for the Felix/Jenna pairing is "Oopsie-shipping".
In Hard Reset, Twilight figures she might as well use a "Groundhog Day" Loop to have a one-night-stand with Princess Luna. In the sequel she finds out she had, long ago, another suitor. One that Twilight's also attached to in a way. Celestia then reminds Twilight that 1,600 years is probably sufficient time. 'You’re probably more closely related to a random unicorn wandering the streets of Canterlot, genetically speaking.'
In A Fvnny Thing Happened on the Way to the Forvm, the heroine is a virgin slavegirl arranged to be sold/married off to the Roman general responsible for having destroyed the land where she grew up. She is somewhat reluctant to marry even before she falls in True Love with the hero. But everything ends happily when a set of matching rings proves the general and his prospective bride are long-lost siblings, enabling the hero and heroine to marry.
Inverted in Ator The Fighting Eagle: Ator and his sister Sunya are "in love" and want to marry, only later to find out that Ator was adopted, meaning they aren't siblings at all and can now safely marry.
The movie My Chauffeur flirted with this. As one of several seeming reveals about who the female lead's "real father" was. Apparently Mom got around.
Oldboy, in which the main character is the victim of an elaborate revenge/imprisonment plot, the final result of which leads him to discover that the woman he has fallen madly in love with is, in fact, his daughter. He, uh, doesn't take it so well.
This is the whole plot behind Say It Isnt So. Faux-Farrely hijinks ensue.
The major couple in The Beastmaster turn out to be cousins, though not one person ever remarks on this. Considering that she's described as his half-brother's cousin, maybe they're not blood cousins after all. She could always be the niece of the king's second wife.
The Japanese film Bubble He No Go has a father sending his daughter (who doesn't know he's her father) back in time some 20 years to find her mother, only to find her younger father instead, who tries to shag her, but she refuses, only to almost give in but then he shortly before realizes that... and so on.
Parodied in Joe Dirt—in Joe's quest for his lost family, he finds and goes out with an attractive girl who he realizes afterwards was his sister, goes back to tell her and explain why he can't date her (and probably ask her for information), but somehow gets seduced into sleeping with her, then tells her his suspicion. She then explains why she couldn't possibly be his sister, and that they can make love again. Joe puts on a mischievous face and the camera cuts back to the outside of the house... as nothing happens.
Jill: Would it help if you went back to thinking I'm your sister? Joe: What?! Like I'm some sort of white-trash perv?! Cut to the outside of the house, the foundations shaking and a nailed-up hubcap clattering off to orgasmic cries: Jill: I'm your sister! I'M YOUR SISTER! Joe: AAAAAAAAAHHHH YER MY SISTER! Joe then assures the radio people he's telling the story to "No, that last part didn't really happen, I just thought it'd be funny."
Becomes a main plot arc in Incendies when the twins discover when their mother sent them to find a lost brother and their lost father, that the person is in fact the same!!!
Code 46 deals with a future where, due to extensive cloning, there is a likelihood of a close genetic relationship between perfect strangers. Maria becomes pregnant by William, only to discover that, biologically, she's his mother; due to the laws, the baby is aborted and Maria's memories are modified to forget she ever met William, but unfortunately, nobody told William. He finds her again, only this time their intimacy triggers an implanted suggestion, causing Maria to turn herself and William in, this time resulting in a memory wipe for him and for her exile with her memories intact.
Played for laughs in Dirty Work: the two best-friend protagonists find out they are actually half-brothers. One of them becomes upset with the revelation and the other cheers him up by reminiscing about their childhood. At first it goes fine, until...
Mitch: Hey, hey! Hey, you remember in 5th grade when I was under the monkey bars and I sneaked a peek at your sister's underwear? Remember that? Hey, no no! I was sneaking a peek at my own sister's underwear!
Sam: That's right! Yeah, and then remember in the 12th grade, you had sex with her?
In Lone Startwo characters that had dated in high school over their parents' STRONG objections reconnect as adults. They later find a photograph of his dad and her mom and realize why their parents were so opposed to them dating, namely, that they're half-siblings. But they're very much in love and she says that, due to complications in her last pregnancy, she can no longer have children. It's implied that they will stay together.
In Tromeo And Juliet, the eponymous pair find out at the end they're brother and sister, but decide to get married anyway. In true Troma fashion, they have hideously deformed kids.
Kind of inverted in Meet the Robinsons. Louis goes to the future, spends time with the Robinson family, and begins to see Franny as a bit of a mother figure (Even actually calling her mom and almost being adopted by her at one point) before finding out that she is actually his future wife. One wonders how he wasn't the least bit squicked out when he meets her for the "first time" in the past.
In the film Jean de Florette Papet and his nephew Ugolin drive a hunchback to his death in order to get his land. In the sequel Manon of the Spring, Ugolin sees the hunchback's daughter Manon bathing in a pond and falls desperately in love with her. He commits suicide when she rejects him. At the end his uncle Papet learns that Manon's father was his long-lost son. That means Ugolin was in love with his own cousin.
In the Michael J. Fox film, The Secret Of My Success, Brantley is asked to escort the wife of his boss to her country home. She takes an immediate liking to him and convinces him to stay for a swim, which leads to her seducing him. Not long after, Brantley realizes that she is his Aunt Vera (by marriage, as his boss is his blood-related uncle, Mr. Prescott). Brantley is disgusted, but Vera isn't - she later pursues him sexually.
Invoked in the Spanish film Km. 0 - a woman hires a gigolo for an extra-marital fling, then comes to believe he's the son she gave up for adoption years earlier. The actual son turns out to be the gigolo's roommate, to her great relief.
In Zathura, Lisa immediately starts harboring a crush on the astronaut once she awakens after having been being frozen for most of the movie. He tries to brush off her advances, presumably because she's jailbait. Later in the film it's revealed that the Astronaut is actually her little brother Walter from an alternate future. She's... less than pleased. (Since it's a space-themed sci-fi film, this could debatably be considered a loose Star Wars reference.)
In the film An Awfully Big Adventure, P. L. O'Hara, an aging actor, seduces a teenage girl, only to find out later that she's his daughter. The distraction caused by this discovery leads to his death.
In Angel Heart, Harold Angel is investigating the disappearance of Johnny Favorite. In the process, he has sex with Johnny's biracial daughter Epiphany Proudfoot. It turns out that he himself is Johnny later on. To make matters worse, he later killed her while under Satan's influence.
In War, Inc., Yonica keeps hitting on Hauser, which creeps him out because she's a teen and he's an adult. At the end, they find out they're father and daughter.
In Soapdish, Jeffery starts dating his ex-girlfriend Celeste's young protege Lori. Celeste strongly objects to this, but they shrug it off as jealousy. Celeste is finally forced to admit that Lori is their daughter. Celeste and Jeffery separated a long time ago when she didn't know she was pregnant. While raising Lori, she told her she was adopted. Jeffery and Lori are horrified, but glad they were stopped before they could kiss.
In the extended cut of Wild Things, Suzie is revealed to be Kelly Van Ryan's aunt. As part of Suzie's scheme to con her older half-sister Sandra Van Ryan (Kelly's mom) out of millions of dollars, she knowingly had sex with her own niece several times. It's subverted in that no one except Suzie was ever aware of this and Kelly never learns the truth.
In Frances Burney's 1778 novel Evelina, the eponymous heroine saves one Mr. MacCartney from suicide, since he believes he fell in love with his sister. MacCartney grows to idolize his savior Evelina in a quasi-romantic fashion. However, at the end of the novel, Evelina is revealed to be his real sister, while his original beloved was an unwitting (and unrelated) impostor.
In "The Maias", by Eça de Queiroz, Pedro goes a long way until finding out Maria, the married woman he's been obssessing over, is his long-lost sister (not even half-sister or anything). Not that it matters to him.
Henry Fielding seems to like to subvert this trope for particular effect: In Joseph Andrews, the hero thinks for a while that his childhood sweetheart is his sister; while in The History of Tom Jones, A Foundling, a random one night stand is later seemingly revealed to be the protagonist's mother... Oh NO! Our hero has done something legitimately bad! Fortunately, this is later resolved to be a lie.
Pyramids by Terry Pratchett features a pair of romantically inclined half-siblings who eventually figure it out (before getting seriously involved). This one might be a Shout-Out to Star Wars, as the non-related male love interest is a Loveable Rogue smuggler with a Cool Ship. (Also there's a bit of a twist, what with them being royalty in Fantasy Counterpart Egypt, where royalty traditionally marry relatives, so the Morpork-educated brother is probably the only person in the kingdom who has a problem with it.)
In Orson Scott Card's Speaker for the Dead, Miro discovers his mom was cheating on his (sterile) dad with her old flame Libo, which makes him and his girlfriend half-siblings. Miro doesn't care, but the girl does. Tragedy ensues.
In the Peter David novel Sir Apropos of Nothing, the title character eventually falls in love with the princess he's guarding. But the morning after they've consummated their love...he finds a flame birthmark on her hip, which suspiciously looks like the one on his. Further, it's revealed (though to Apropos only) that the princess is the illegitimate daughter of the Queen and the court jester... and that answers Apropos' question of who his father is, as his mother was raped by a gang of knights... and a shadowy figure that matches the description of the court jester.
J. R. R. Tolkien's The Silmarillion includes the tale of Túrin Turambar (dealt with at greater length in The Children of Húrin), whose sister Niënor was born after he left the family, causing them not to meet until she, having gone to look for him, has had her memory erased by a dragon. Therefore, not knowing who she is, they get married and she even gets pregnant, but then the dragon restores her memory and she promptly commits suicide. Túrin does the same shortly afterwards. Based on the story of Kullervo in The Kalevala.
In One Hundred Years of Solitude, the biggest fear of the matriarch Ursula is that some of her descendants will be born with a pig's tail, in the fear that it could be the end of both her family and the town. This finally happens a long time after her death, when the last members of the family, Amaranta Ursula and Aureliano Buendía, aunt and nephew respectively, mate and give birth to a pig-tailed baby who is eaten by ants, after his mother dies in childbirth and his father forgot to take care of him because of his deep grief. Both lovers were unaware of their real relation, and Aureliano Buendía only discovers it in the last page of the novel, just seconds before the destruction of the town.
In Poppy Z. Brite's novel, Lost Souls, a teenage runaway called Nothing meets up with a trio of real vampires led by the captivating Zillah, and becomes Zillah's lover. Halfway through the novel, it's revealed that Zillah is Nothing's father.
In Thieves' Worldbadasses Tempus and Cime seem to be laboring under various curses related to this (or effects of mitigators). It seems to have driven them both Axe Crazy, though in different ways. Some early stories danced around whether they really were related, but Janet Morris' A Man and His God confirmed it.
In the novel The Shadow of the Wind, Julian, an illegitimate child who doesn't know who his father is, has a secret relationship with Penelope, the daughter of a rich man who mysteriously decided to take Julian in and pay for his education. It is, however, not until years later, long after Penelope has died giving birth to Julian's child, that he learns that she was actually his half-sister.
In the novel Moll Flanders by Daniel Defoe, Moll unknowingly marries her half-brother (her mother having abandoned her as an infant) and has two children with him. Upon discovering the truth, she dissolves the marriage and flees Virginia for England, leaving her two children behind.
Almost used later when Gendry, who has no clue that his father is King Robert, meets a whore who flirts with him, all the while bragging that she's the king's bastard. Part of her argument is that she has his famous black hair. Arya (who does not know either) argues that Gendry has it, too, so it's not like it means much. Fortunately for everyone, Gendry's really not in the mood.
In Robert A. Heinlein's Friday, the eponymous Friday has a bit of a crush on her father-figure employer, who is later revealed to actually be her father... or at least a male who contributed some of her DNA, Friday's genealogy being somewhat non-traditional.
In Juliet Marillier's novel Son of the Shadows, the protagonists' sister, Niamh, falls for a druid. When her family finds out, they marry her off in a strategic alliance without telling her that the reason they're so horrified is because the druid (Ciaran) is actually the son of her mother's evil stepmother, thus making him her half-uncle. Oops.
Their relationship is later vindicated though. Their daughter turns out to be the child of the prophecy, ending a longstanding feud with Britain over sacred ground.. Unfortunately, Niamh never lived to see this, having been pushed off a cliff by the aforementioned evil stepmother when the child was a toddler.
When you look at their family tree, Liadan and Bran/The Chief are second cousins.
In Lirael by Garth Nix, Prince Sameth shows some interest in Lirael, without knowing she's his aunt. At the time she didn't know either, but she's not interested and puts him off by telling him that she's actually thirty-five but has a marvelous skincare regimen. Later, Nick just assumes they're engaged when he sees them together, and when he gets set straight and says she looks too young to be an aunt, Lirael hastily mutters that she's much older than she looks.
In Kafka on the Shore by Haruki Murakami, the protagonist begins to have a sexual relationship with an older woman who he suspects might be his mother. She denies it, but it is heavily implied both that she actually is his mom and that she already knows this to be true.
In The Hollow Hills, the second book in Mary Stewart's retelling of the story of King Arthur, Morgause, Arthur's half-sister, bewitches him into sleeping with her, and he only finds out afterward of their relation to one another.
Subverted in The Elvenbane by Andre Norton and Mercedes Lackey when Valyn is determined to push away Shana even though he admits that she is lovely and knows that she has a massive crush on him. He does this because he suspects that she is his half-sister. He actually goes so far as to set her up with his sidekick in a "handfasting" ritual. However, he picks up the Idiot Ball when he refuses to tell her why he is rejecting her even though she is hurt and angry about it. His excuse? She was Raised by Wolves (or dragons, in her case) and he's not sure if such things matter to her, in spite of the fact that she has already shown herself to be intelligent and fairly well-versed in human and elven culture.
In the first trilogy of The Chronicles of Thomas Covenant Thomas is the object of open sexual advances by High Lord Elena, who he eventually realises is biologically his daughter, though (thanks to the time discrepancy between the Land and 'our' world) she actually appears to be older than him. Unsettlingly, she knew all along. Though this discovery explains why he finds her emotionally compelling, he continues to wrestle with having some degree of physical attraction to her - made all the more complicated by the fact that he believes his entire experience is a delusion.
In the Jin Yong novel Demi-Gods and Semi-Devils, all the women Prince Duan Yu has relationship with turned out to be one of his father's natural daughters. Subverted in that Duan Yu was later revealed to be the product of a one-night-stand between his mother and a rival to his father's throne.
The Kwisatz Haderach breeding program of the Bene Gesserit in the Dune series does rather a lot of this if the crosses mentioned in the books are a representative sample. The people involved are generally unaware, at least at the time.
According to their plan, Jessica was supposed to have a daughter with Duke Leto Atreides. The daughter was then supposed to be married off to a Harkonnen heir in order to end their millennia-long rivalry. However, Jessica is actually Baron Vladimir Harkonnen's daughter (although she didn't know until Paul told her). This means that the plan was for Jessica's daughter to marry her cousin.
The incest is averted in Havemercy, but just barely. Word of God has it that the authors had the brothers in question unrelated in an earlier draft, and deliberately included plenty of sexual tension and the possibility of a relationship between them. Later, they decided to take the relationship in a different direction, and proceeded to erase all hints of UST between them. They were fairly unsuccessful, though, as most of the readers picked up on it anyway. Since the fact that they're brothers is hidden until near the end of the book, almost all of the readers were already shipping them by default by the time they found out. And once the ship has sailed, well, there's no calling it back to shore.
Happens not once, but twice in The Stone Dance of the Chameleon. Carnelian and Osidian meet in secret and have a passionate affair. Then it turns out that Osidian is the God Emperor's son, making them cousins, since Carnelian's mother is the emperor's sister. Later, though, it turns out that Carnelian's father is actually the emperor himself - which is not atypical in the ruling house, Osidian's mother being the emperor's other sister. Making them, you know, brothers. In case you hadn't noticed, this is one messed up family.
In the crime novel Lie to Me by David Martin a man kills himself rather messily after finding out that his young wife is really his long-lost daughter. He is particularly distressed by the revelation that she always knew he was her father, but married him anyway.
The Monk: As if raping Antonia wasn't enough, Ambrosio later learns she is his much younger sister.
In the final Lensman novel, Children of the Lens, it's all but stated that the Kinnison kids (one boy, four girls) will be the progenitors of the Arisians' replacements.
In Sherlock in Love, Sherlock Holmes finds himself falling in love with the astoundingly intelligent and musically-gifted Violet Sigerson. And then finds out that she is his half-sister, due to his mother's marital unfaithfulness. Holmes eventually tries to pursue a relationship, but Violet will have none of it and fakes her death to stop him forever.
In A Brother's Price, Jerin and Ren hit it off to the point where he's left a Technical Virgin. Then they find out that because his grandmothers kidnapped a prince, they share great-great-grandmothers, making them first cousins twice removed, second cousins once removed, or third cousins. Jerin is initially disturbed by this revelation, but Ren says it's fine and later others agree. Many of the nobility are somewhat more inbred than commoners, and actually this reveal makes him a Suddenly Suitable Suitor.
Max Frisch’s Homo Faber. The novel is mostly just the narrator whinging that he really, really couldn’t have know that the young woman he slept with was his daughter.
At the end of the Marquis de Sade's story "Florville and Courval" (from his collection The Crimes of Love) the heroine discovers that she has unknowingly had a child by her brother, killed that child in self defense when he was trying to take her sexually, and married her father.
Severian of The Book of the New Sun never knew who his family was, so this was always possible given the number of women he took to bed. At the end of Citadel he discovers that Dorcas is actually his grandmother, who had been drowned decades ago and then returned to life by the Claw. He notes that he still loves her, but she loves the past more than him.
In Remember Me, by Christopher Pike, Shari's brother, Jim, is dating a girl named Amanda. It turns out that in fact Jim and Amanda are brother and sister.
The plot of "Lace" by Shirley Conran revolves around a movie star named Lily gathering four older women (who have been best friends since boarding school), demanding to know which one of them is her mother. As it turns out, one of the women got pregnant as a teenager and had to place her child in foster care but for various reasons her daughter ended up lost and she had no way of finding her. As the story is told the reader doesn't know yet who the mother is and since Lily at one point met and deflowered the son of one of her potential mothers you're left to worry that she might have had sex with her half brother. In the end it turns out that her mother, Judy, never had any other children so Lily hasn't slept with a sibling. Instead only Judy realizes the true squick factor. When asked who Lily's father is she lies and claims it was a male friend of the girls but in truth she was raped by an Arabic prince and became pregnant. That same Arabic prince later took Lily for a mistress and they spent a couple of years having the best sex of their lives with one another. He never knew she was his daughter and because the affair was public knowledge Judy decided to keep the truth from Lily to avoid traumatizing her.
Live Action TV
An unusual example in True Blood, in which vampire Bill finds out his human squeeze Portia is actually his 3rd great-granddaughter, but Bill is the only one who's grossed out by this. Portia is more than willing to continue their relationship (and great sex), citing logical reasons why this shouldn't affect them (e.g. their genetic differences are about the same as with any other person, there's no risk of children since he's a vampire, most states are rethinking their cousin marrying laws). Bill then glamors her into having a screaming fit every time she sees him. Maybe in addition to the incest he didn't want to be Sheriff Andy Bellefleur's brother-in-law.
Narrowly averted when Ste meets his estranged father Danny, and heavily flirts with him before finding out they are related.
Dodger turns out to be the father of his own twin sister's daughter - they had been separated as children, then met again and slept together as teenagers without being aware of each other's identity.
Occurred in at least one plotline in Law & Order: SVU The girl was the product of her mother's affair with the boy's father, and when the two began dating due to accidental reverse psychology (the mothers hated each other - the father was in fact the husband of both and living a double-life, known about by one of the wives the whole time) the girl is accidentally killed by her father when he reveals to her that he is the boy's father too. The autopsy reveals that the girl was pregnant when she died and that the boy was the father and that he was related to her. The boy is horrified and vomits in Stabler's office. The boy's mother kills his father when she finds out that he was planning on leaving her for the girl's mother, who was jailed for her part in the myriad crimes going around, with whom he also has a son, the boy's half-brother, and leaving the two new half brothers without any parent not in prison.
One episode of House had a young married couple who both turned out to share the same rare genetic disease... because unknown to the couple, the man's father had slept with the woman's mother and they were half-siblings.
On top of that, the half-brother wants to stay together; while the half-sister is disgusted. The episode ends without resolving the situation, leaving the brother/husband hoping that she'll come back while Foreman (who supports the relationship) keeps him company.
Hispanic soap operas used to love this so much, but it has faded in recent years. The ultimate parody of how this used to be done is the song "María Teresa y Danilo", where a girl, daughter of the aforementioned characters, is forced to cancel her wedding after discovering that her fiancé is the son of her father... only for it to be revealed by her mother that, in fact, she is not his biological daughter.
Subverted in one memorable Argentinian case: the main characters find out they're siblings and go their separate ways. As usual, it turns out to be a lie. Only it's actually true... and their father has decided not to tell them. They marry and live happily ever after.
As the World Turns: Holden and Lily are one of the show's biggest supercouples. Lily is also the biological daughter of Holden's sister Iva. Who fortunately turned out to be adopted herself so it was not incest after all.
Red Dwarf: it was revealed that Lister's ex-girlfriend Kochanski is actually his mother. In the next series, Lister still uses a sexual-magnetism virus to seduce her in an elevator despite his knowledge of this and it nearly works. Then again, Red Dwarf is not known for its continuity, and the revelation of Lister's parentage was probably not carried over from one series to the next. This editor certainly hopes so.
Then again, Lister also discovered at the same time that he was his own father (via in-vitro fertilization), which probably trumps any genetic relationship concerns.
Cat, and his species, were descended from one pregnant housecat. Just as well that he was the last of his kind and didn't have anyone to mate with...
They also could also be a race of Half-Siblings if you take into consideration that female cats can actually get pregnant from two (or more) different males in a single litter. However, the book makes it clear that the reason the cat population didn't crash due to genetic bottleneck was due to the large amounts of background radiation promoting mutations.
Anticipated by Rimmer in "Back To Earth"; After telling the creator to create for him a beautiful, adoring girlfriend, he quickly adds that she should NOT turn out to be his long-lost sister.
The novelization Better Than Life has Rimmer's second wife turn out to be based on a younger version of his mother. He can't even complete the thought when he realizes it, he's too disgusted and horrified.
In the 2007 remake of Flash GordonAura and Terek were in place to start up a romantic relationship, only for the eponymous hero to burst into the room to inform them that they are siblings. Later, it turns out they're twins.
On Just Shoot Me!, Jack's handsome new ex-marine trainer keeps trying to ask Nina out. Nina, who usually Really Gets Around, keeps refusing, much to her surprise. It is at episode's end that she discovers why she's not attracted to him: they're cousins.
In another episode an old female friend of Jack's shows up with her son, and he and Maya show signs of being interested in each other. Jack, however, is horrified, because he had an affair with this friend right before her wedding and the timing would line up with him being the young man's father. Eventually she reveals that she secretly got a sample of Jack's DNA years ago and tested it, showing he's not the father. By this point, however, Nina's broken the news to the would-be couple, and the man is horrified.
In the 30 Rock episode "The Head and the Hair", Liz and the Hair find out right before kissing that their grandparents are cousins. He seems a little more comfortable with it than she is:
The Hair: I think we're third cousins. Liz: Yeah, I'll see you at the reunion.
On Grounded For Life, Lily made out with a guy at the funeral. She later learns he was her cousin. Then she is somewhat relived when she learns he was a cousin by marriage but squicked again when learning they're cousins by his marriage.
On Green Wing, Guy Secretan doesn't find out Joanna is his long-lost and presumed-dead mother until after they've slept together. And it's been caught on tape.
The Korean soap opera Winter Sonata is Marmalade Boy if Yuu went missing, got amnesia, and really is half-siblings with his love interest. Well, in fact he could be instead the half-sibling of his love rival, but the soap just was happy toying with that until the last chapter.
There is a memorable scene in this drama where one of the rivals, after being clued of the above situation and arguably tired for the on-off situation who has already left her out of the game, ends yelling to one part of the conflicted couple "It doesn't matter if you are really siblings or not, just go out of the country and marry anyway! Abroad nobody is going to know!".
In Veronica Mars, Duncan Kane broke up with Veronica because his mother told him she was his half-sister. After the first season finale revealed Veronica and Duncan weren't related, they got back together (until Duncan got Put on a Bus).
In the Dollhouse episode "Haunted", Echo gets imprinted with the personality of a wealthy woman who wants to attend her own funeral. The woman's son makes a pass at her; she's suitably disgusted.
NCIS season 6 episode "Heartland" had the team go to the mining town of Stillwater (coincidentally Gibbs' birthplace) where it was revealed the hospitalized Marine and his childhood friend, who remained in Stillwater waiting for him, shared the same father unbeknownst to them both.
Though it's worth noting that she maintains that their relationship was strictly platonic, though they did have feelings for one another.
On Nip/Tuck, the Carver and the female police officer investigating the case are siblings. Who are sleeping together. In season 5, Christian finds out that he fathered a daughter when he was in college. She just happens to be Matt's new girlfriend.
Carnivàle: All right, we'll be fair: Justin Crowe wasn't actually aware that he'd made creepy passes at two close female relatives. After all, only his sister-lover actually knew that the new maid was Justin's illegitimate daughter. Not that that necessarily would have stopped him. At all.
On the British soap Emmerdale Maisie Wylde and Ryan Lamb are caught in bed together, and are promptly informed (after many months of hiding this from them) that they are half siblings. Which disgusts both of them and wind up sending Maisie into rehab and getting their father shot.
On MTV's late-night, sex-advice talk show Loveline, a man called in asking what to do after discovering his pregnant girlfriend is his half-sister (his mother was estranged for years and the girl was racially-mixed, so he had no idea at first).
On Smallville, when Clark meets Kara (aka Supergirl), they seemed a little attracted to each other. When Kara's comment about her father Zor-El causes Clark to realize that they are cousins, he backs up, clearly (and understandably) disappointed.
Kamen Rider Double has a strange example that's not quite playing with and may be a Relationship Writing Fumble or may have been intentional. Phillip is really the brother of Wakana but doesn't know it at the time. He's close to her and plans to leave town with her. Then it turns out that she's really his sister. Once the reveal is made there's no change in plans or attitudes at all (although things change for other reasons) and the audience is apparently supposed to accept that "I care for this girl I know" and "I care for my sister" were exactly the same thing all along. There isn't even a Lampshading. Although, to be fair, he never really seemed to think of her that way. Or any other girl or guy for that matter.
Any Ryoutarou/Hana fics that were ongoing when it was revealed that she was his niece had to take this route. At least they got off better than the Yuuto/Hana shippers ...
Kamen Rider Kabuto has plenty of Ship Tease for Tendou and Hiyori earlier in the series (with a guest actor who'd played Tendou as a child stating in an interview that he'd been led to believe Hiyori was Tendou's love interest.) However, Hiyori turns out to be a Worm that has copied itself from Tendou's sister, making them siblings. Subsequently, the show dropped any potential romantic arc between them and began having Kagami pursue her instead.
On Dallas, the writers revealed after a few seasons that the Ewings' farmhand Ray was Jock Ewing's illegitimate son, having forgotten that in the beginning of the series, he had been having an affair with Jock's granddaughter Lucy. After realizing what they'd done, they made sure to never reference the affair again, though it was never actually denied to have happened.
In a Season 5 episode of The A-Team, "Family Reunion," Face helps an attractive woman come to terms with her fugitive father reentering her life. Afterwards, he tells Murdock that he's going to ask her out after their mission is over because he's never met anyone else he's "felt so at ease with." Murdock starts insisting that she's not Face's type because he and the audience both know, due to dramatic irony, that this beautiful woman is actually his half-sister, and her fugitive father is also his.
In season 3 of Sons of Anarchy it is revealed that Jax Teller and Trinity Ashbey are both John Teller's children. Ony a few people know of this and since Jax lives in California and Trinity in Northern Ireland they do not see the need to tell them. When Jax comes to Ireland he and Trinity quickly hit it off and are about to have sex when their mothers walk in on them.
This was in an episode of Midsomer Murders a man had 'spread his seed far and wide'—you could hardly turn a corner without finding one of his bastards. One couple didn't meet until they were both in graduate school in Canada and got married, only later realizing they were half-siblings; she was squicked, he didn't mind. It's also been used in several other episodes.
On Gilmore Girls, after Lorelai's grandmother dies she asks Emily what her maiden name was, and Emily explains that it was already Gilmore because she and Lorelai's grandfather were second cousins. Lorelai and Rory find this information horrifying, while Emily just notes that it used to be a common custom in well-off families.
Endgame: When the son of the Huxley Hotel's owner goes missing, it turns out that he's gone on a bender because he discovered that the girl he was planning to marry was really his half sister, because his father had had an affair with her mother. but then it also turns out that his mother had been having an affair too, so his father wasn't really his father, and they were unrelated after all.
Zig-zagged on Arrested Development; Maeby has an on/off relationship with Steve Holt, who is ultimately revealed to be her uncle GOB's illegitimate son (Played Straight). This prompts the couple to break up, but it turns out that Maeby's mother Lindsay was adopted, meaning Maeby isn't related to Steve after all (Subverted), or indeed her other cousin George Michael, who had been attracted to her since the pilot (Inverted). An adult eventually finds out, however, and decides that regardless of who is related to whom the whole thing is just too weird.
In one episode of The Love Boat, a woman comes on board with her son, to meet the daughter she gave up for adoption. Before they all get together, the son and the daughter meet, and, not knowing who the other is, they fall in love and have sex. The daughter is freaked out when she meets the mother and discovers who the guy she slept with is. Fortunately the son is actually the mother's step-son, so the couple aren't blood siblings and can get married, but the mother has no idea how she'll phrase the wedding invitations.
In The Nanny, Fran finally meets a guy who seems absolutely perfect for her: Jewish, a doctor, and understands her. At Fran's cousin's wedding, they see each other, and kiss. Cue Fran's mother...
Sylvia: Isn't that adorable. Let me get a picture of you and your cousin Bob.
(Fran and said cousin immediately jump apart, looking thoroughly Squicked)
Bored to Death, Jonathon and his girlfriend find out they're half-siblings. Left unresolved due to cancellation.
In a more minor example, both Cory and Eric leer at an old picture in their parents' photo album of a sexy young woman in a skimpy wet outfit, only to freak out when they realize it's their own mother.
A subtle one occurs in the relaunch of Battlestar Galactica where Ellen Tigh sleeps with Cavil to free Saul. As it turned out, Ellen was one of the Final Five and was essentially Cavil's mother.
It was a double-whammy, too. Not only was Cavil designed to resemble Ellen's father, she had also viewed him a son. Just another sign of how deep his misanthropism and hatred toward his "parents" really ran.
The end of the episode with the Pimp Hat in Less Than Perfect had Owen and his Distaff Counterpart date reveal that one of the many things they had in common was that they both had donor dads who were artists. Claude pulls him aside to tell him that it was very likely they were brother and sister. He retorts that "this couldn't wait until tomorrow?", meaning he wasn't completely against the incest.
This becomes a motive for murder on a case in CSI: Miami. Dad had a Secret Other Family, son from Family A met up with daughter from Family B, fell in love, and had a baby together. Then son found out...
In Friends Ross and Monica, who are siblings, kissed when both were drunk at a party. However, Monica was half-asleep on a bed and didn't know who that kiss was with, and Ross thought she was Rachel under a pile of coats as it was in a dark room. They only realized it was them years later.
Ross: You were my first kiss with Rachel?
Monica: You were my first kiss ever?
Chandler: What have I married into?
In Scrubs when JD's girlfriend gets pregnant, they discuss the situation and JD is opposed to adoption because he fears this trope may occur if they have a daughter.
In Lewis Brother and Sister twins that were adopted by different families meet, fall in love, and marry. When they can't have kids , the couple gets genetic test done. They find out not only that they are related, but they have both have the markers for Fatal Familia Insomnia. They track down their birth mother but get Dr. Dobson's name because their mom used it. They are slowly going insane from lack of sleep.
A common Plot Twist in soap operas is that some couple found out they are somehow related. Did I say common? Super commonPlot Twist.
Days of Our Lives had a scene in which Lucas almost has sex with Cassie. They are only stopped when Rex, Cassie's twin brother, arrives and tells them that they are half-siblings.
Before that, Cassie had a crush on and kissed her cousin Shawn-Douglas.
Long before that in Days, Tony DiMera was in love with a woman named Renee. Unfortunately, it turned out they shared the same father, Stefano, but luckily it soon came out that Tony was really the son of the family's gardener, not Stefano (of course, since this was a soap by that point Renee had married another man...and then got killed by a serial killer...). For long-time fans, this caused a serious amount of squick when years later a retcon made Tony Stefano's biological son, but in the end another retcon made the Tony that had appeared since then his evil doppleganger Andre (long story, naturally) and showed that Tony wasn't Stefano's biological son after all.
Arrow: Early scenes suggesting a possible spark between Tommy and Thea become squicky in hindsight after the Season 2 reveal that they are half-siblings, sharing the same father. (Though Tommy was dead by the time it was revealed, making it a moot point anyway)
In Lost Girl, Bo gets hit on and kissed by a woman who turns out to be her long-lost mother (Bo and her mother are succubi and don't age).
Robin and Barney from How I Met Your Mother were on their way to their wedding when they discovered they were related through a mutual cousin named Mitch. By the end of the episode, it's revealed that Robin at least is not blood-related to Mitch, so they manage to avert this trope. They did spend the entire episode awkwardly avoiding any sort of contact though.
Game of Thrones has a somewhat squick-worthy one-sided example of this. In Episode 2 of Season 2, Theon Greyjoy returns to his homeland where he meets a young woman and they begin flirting. During a subsequent horseback ride, Theon begins feeling up the woman in a sexual manner (this is the series that led to the phrase "sexposition" being coined) and she not only doesn't object, she seems to take pleasure from it. A few minutes later, Theon (and the audience) get squicked when he learns that woman he was making out with was his sister, Yara, whom he hasn't seen since she was a child, and that she was aware the entire time and played along mostly to see how he'd react in a Secret Test of Character. Needless to say, he failed miserably.
Inspector George Gently: "Gently Going Under" features a half-siblings version. The girl's father had an affair with the boy's mother while her husband was away at the war. When the relationship is discovered, the father tries to move away and take the daughter with him but she is already pregnant. The boy's mother tells her the truth, causing her to attempt to abort the child.
In EastEnders, Kareena Ferreira dated family friend Tariq, later making the relationship incestuous when it's revealed Tariq is her half-brother; the family's father had an affair with Tariq's mother. Tariq knew the truth, but thought that he could get closer to the family by dating Kareena, taking care to ensure the relationship never became sexual.
The big reveal of Type O Negative's "Stay out of my dreams" is this trope.
With your straight black hair and emerald green eyes Hippies pointing "that's Pete's sister in disguise" Maybe you had uttered those words as a jest I don't mind the allegations of incest At the time I thought I could love no other Till I heard you say that I was your brother In your e-mail said how much we look like twins How it turned you on just knowing it's a sin
"Weird Al" Yankovic, A Complicated Song - "How was I supposed to know / we were both related? (Believe me if I) / Knew she was my cousin / we never would have dated".
He then goes on to wonder what he should do: dump her or get married and move to Alabama, where they can raise kids with 11 toes.
The song "Son Don't Go Near the Indians"- The whole time the main character of the Western song is told "Son don't go near the Indians, please stay away." Reverse psychology plays its part, and he goes near the Indians—a lot. He falls in love with a girl there about his age. Near the end, they resolve to get married. Only then does his father explain that his actual son got killed by the Indians, so he stole one of their baby boys, the protagonist, and that his girlfriend is his sister. "And that's why I've always said—'Son, don't go near the Indians, please stay away'"
Subverted in the related song (most likely a parody) "Son Don't Go Near the Eskimo" which leads the listener to suspect a similar conclusion, making the big reveal (the girlfriend is not his sister, but she has "the coldest nose in Alaska") somewhat anticlimactic.
In the folk song "I'm My Own Grandpa," the narrator marries an older woman with an adult daughter. The narrator's father then marries the daughter, making the son's stepdaughter his stepmother. Because the narrator is married to his stepmother's mother, he has become his own step-grandfather. Not actually incest, either.
A real life example was just avoided when Mandy Smith's mother broke up with Rolling Stone Bill Wyman's son. Smith and Wyman were married at the time, giving Smith a narrow escape from becoming her own step-granddaughter-in-law.
Shame and Scandal subverts this: This young man discovers his dad is also the father of every single one of his girlfriends then learns from his mother that his "dad" his not his real father.
The song "Butterflies" by Sia plays this on the listeners. It sounds like just a regular song about lingering awkwardness in a relationship, until the very last line right at the end of the song, when she casually remarks "Because we came from the same cocoon".
Cornelis Vreeswijk's Incestvisan (Incest Song) has a similar structure an example named above: Boy meets girls, falls in love. Dad intervenes, claiming they are his sisters. Boy then talks to mom, mom: "Oh, don't worry, he's probably right but he's not YOUR father anyway..."
Frank Zappa's "Magdalena" is a lengthy and detailed confessional by a Canadian maple syrup salesman to his teen-age daughter of what he'd like to do to her.
There's also "Brown Shoes Don't Make It", with this exchange, repeated three times:
Man: If she were my daughter I'd...
Girl: What would you do, Daddy?
The protagonist of the Chinese classic Dream of the Red Chamber is first cousins with both of his main love interests, although this was hardly taboo in China at the time.
The folk song "Johnny Be Fair" parodies this. The female narrator repeatedly falls in love with men, only for her father to pull out the Parental Marriage Veto because he's their father too. In the end she goes to her mother, who says "he's not really your father, marry anyone you want". It dates back to Elizabethan times and probably further.
Gothic Americana might not exist as a genre without this trope. Slim Cesna's Auto Club has an EP named "Crossbreeding Begins at Home." Similarly, about half of Jay Munly's oeuvre seems to rely upon this trope. If you're curious, look up the lyrics to "Big Black Bull Comes Like a Caesar" and "There's a Goose Walking Over My Grave."
Tom Lehrer wrote a jaunty, upbeat "theme song" for Oedipus Rex.
When he found what he had done, he tore his eyes out, one by one. A tragic end to a loyal son who ...loved his mother.
Harry Chapin's "The Mayor of Candor Lied" is a particularly bone-chilling twist on this one: boy meets girl, girl's father disapproves, ostensibly because the boy is poor. Boy catches his mother with her father and blackmails him into letting boy and girl be together. Then her father takes her overseas on vacation and doesn't bring her back, and in the ensuing confrontation, the boy suddenly notices how much the girl's father looks like him...
In The Saga of Hrolf Kraki, King Helgi of Denmark marries the servant girl Yrsa whom he kidnapped on a viking raid to Saxony. It eventually turns out she is his own daughter he conceived by rape on an earlier raid.
In The Saga of the Volsungs, Sigmund sleeps with a vagrant soothsaying woman. Many years later, Sigmund learns the soothsayer was his sister Signy in disguise, and her son Sinfjotli who Sigmund thought was his nephew is actually (also) his son.
In The Marriage of Figaro, the character interested in marrying Figaro turns out to be his mother and says something like she always loved him just in the wrong way previously.
The Ring Cycle. In which Siegmund gets away with Sieglinde, who's his twin sister, and they become the parents of Siegfried, the ultimate superhero. Later, Siegfried marries his aunt, Brünnhilde... (Both the twins and Brünnhilde are Wotan's children. Don't even make me start about Wotan's love interests, he's a Casanova.) And in some performances, the Gibichungs are pretty suspicious too.
As Anna Russell puts it, though, Gutrune Gibich is the only woman Siegfried has ever met, in his entire life, who wasn't his aunt.
"Emmeline" In which the eponymous 19th century New England woman has an illegitimate son who is taken away from her just after birth. The son is given up for adoption and raised by a family in Ohio; Emmeline, meanwhile, saw the baby for such a short period of time, she is easily convinced by relatives that she had a daughter, not a son. Twenty years go by, and Matthew, a handsome young laborer, appears in Emmeline's hometown. They unwittingly fall in love and marry. Later, a member of Emmeline's extended family shows up to attend a funeral, and, while there, recognizes Matthew as the child Emmeline gave up long ago. After this earth-shattering reveal, Emmeline is abandoned by her son and made to be a pariah in her community, where she lives the rest of her life in miserable isolation and shame.
The Roman comedy trope used in A Funny Thing Happened to Me On the Way to the Forum was used more seriously in Gotthold Ephraim Lessing's play Nathan The Wise- the title character is Jewish and it is revealed that his daughter is actually not a blood relative and he is charged with the crime of raising a Christian as a Jew. It ends up turning out that she is the sister of a literal and celibate Knight Templar in play (taking the place of the mercenary of Roman comedy), who other characters had been trying to fix her up with. Both of them turn out to be the children of Saladin's brother and a Christian woman and in at least one translation, the Templar expressed gratitude at the news, as he was not yet prepared for sexual love.
In The Importance of Being Earnest, it's revealed that Jack and Gwendoline are long lost cousins, but they don't really care and still want to get married anyway, which was not unusual for the time.
Played with in The Guilty Mother, the unpopular Downer Ending segment of the Figarotrilogy. Both lovers are not the biological offspring of both of their legal parents, and Figaro has to juggle their Who's Your Daddy?? plot so that they can marry and nobody needs to be publicly disgraced.
Thomas Middleton's 1611 play No Wit, No Help Like a Woman's plays with this trope: one subplot features a young married couple who are horrified to find out they're actually brother and sister, and then quite relieved to discover that the girl was in fact switched at birth so they're not related at all.
August: Osage County. Little Charles and Ivy have a Kissing Cousins relationship, which is made worse when it's revealed that Little Charles is actually the biological son of Ivy's father, making them brother and sister as well.
Mother: Oh, God! What have you done! Wendla and Melchior: Mother! (To each other.) That's my mother, not yours. Stop doing that! (To Mother.) Mother! Mother: Actually, I am both of your mothers. Wendla and Melchior: Both? But that would mean... eeew! Mother: Let me explain. In some scenes, I am Melchior's mother, and in others, I am Wendla's. I also play a piano teacher, and when I wear this hat, I'm Frau Knuppledick. Four different characters, all wearing the same dress.
In Shadow Hearts: Covenant, the hero Yuri meets the babealicious Karin and the two go through many harrowing adventures together. It becomes clear pretty quickly that Karin is falling for Yuri, although he's still pining over his lost love from the first game in the series. However, at the end due to some messing around with the space-time continuum, Karin ends up traveling back in time and marrying Yuri's father...and becoming Yuri's mother. Good thing they never actually got together, eh?
This makes sense in a twisted way. Considering that Yuri and his father were apparently very similar people, it does make sense that Karin first falls for Yuri in her time and for his father in the past. While they only remained friends, Yuri does manage to convince Karin to wear a very skimpy outfit. The scene become either squicky or more hilarious afterwards depending on how perverted your sense of humor is.
In the Visual NovelEver17, Sara is pretty clearly attracted to the Kid, and vice versa... until it turns out that they're long-lost siblings, at which point they switch gears surprisingly smoothly into a platonic relationship, without so much as a moment of disgust over having flirted.
In the Baldur's Gate series, Imoen and the main character (if he's male) have the sort of sorta flirty relationship you'd expect from lifelong friends of opposing genders. For that matter, if the main character's female they have the sorta flirty relationship you'd expect from lifelong friends of that sort. When Baldur's Gate II was released and Imoen was not one of the characters with whom you could pursue a romantic relationship, a lot of fans of the original were very surprised and rather disappointed... until you found out that Imoen was your half-sister.
This is actually brought up in Throne Of Bhaal, the expansion to Baldur's Gate II, if the main character's male. Viconia insists to Imoen that given the number of times he's saved her life, she should serve him in every way. Imoen acts squicked and cites why. Viconia brushes it off, pointing out that the common heritage would hardly one of blood. Imoen then notes that, canonically by that point, the main character has saved Viconia's life at least twice...
Some people tried to do a mod that allows this relationship; the Imoen Romance mod for Baldur's Gate II is alive and well, and is widely known for near-total freedom of choice in the protagonist's options. Imoen is romanceable by both males and females, and to the mod's credit, does not shy away from the incest issue—Imoen in particular has issues with it.
Where to start with Fire Emblem? Well, Genealogy of the Holy War is king. Two of the biggest examples are Seliph and Princess Julia; upon meeting they seem instantly attracted to each other, but it turns out that Julia is Seliph's inbreed half-sister, as Seliph's mother Deirdre was brainwashed and forced to breed with her half-brother Arvis (who had no idea that they were siblings). There's a reason that the game's Relationship Values system won't allow them to be paired...unless you exploit a programming glitch. In the first half, Claude's most popular pairing is with Sylvia, who is hinted to be his long-lost sister, but said by Word of God to be a distant cousin.
Averted with Fire Emblem Awakening, where Chrom can hook up with any female met before chapter 13 (even the Player Character, if she's female), with two exceptions. One is Lissa, who's his sister. The other is a masked figure calling themself Marth, who turns out to be his time-warped daughter Lucina, though he doesn't find out about this until Chapter 13.
Shows up occasionally in fanworks made with The Sims 2: unattended children can be taken away by a social worker, and then adopted into a different family. The adoption code nukes the family connection to the birth parents and creates one with the adoptive parent(s). Some players have used this loophole to have siblings, and twins, adopted into different families, strike up romances. Since there's no genetic penalty for incest, they can even have kids. Read all about it.
In the PS2 adventure game Drakengard , where it is central to the entire plot, even if the English language version of the game has been Bowdlerized into omitting all but two direct references to incest.
In the PS2 adventure game Shadow Of Destiny, with Eike and Dana living happily ever after, despite secretly being biologically father-and-daughter, in Ending E.
Well, there's a reason why it's one of the worst endings. And Dana seems to be at least subconsciously aware of the connection on some level and is bothered by it.
The Casual Video GameVirtual Villagers allows you to control up to ninety individual village-members with their own names, appearance and skills. The females can get knocked-up. By anybody. That includes their brothers, fathers, and offspring (assuming the offspring is old enough). Thankfully, the game doesn't actually keep track of family relations (which is probably why this is possible), sparing the sensibilities of players who forget or don't pay much attention to the lineage. Or turn their back on randy relations for half a second.
Used in No More Heroes when Travis is informed that his ex-girlfriend Jeane, who he did sleep with, was his half-sister, which she knew beforehand. He reacts appropriately.
In Apollo Justice: Ace Attorney, we see the protagonist, Apollo Justice. His attractive assistant is Trucy Wright, Phoenix Wright's daughter. They were often paired together in eyes of the fans, until it was revealed that Trucy was adopted, and her last name is really Enigmar. The problem? That was the maiden name of Apollo's mother, and the two were half-siblings. Oddly, it still hasn't stopped the fan pairing from still occurring.
A good chunk of the reason why the pairing still happens is probably because while the players learn that Trucy and Apollo are related, they themselves actually do not find this out, oddly enough.
In the first Killer Instinct game, Black Orchid has a finishing move where she unzips the front of her outfit and flashes her breasts to her opponent, causing male characters (including Jago) to pass out in excitement. At the end of the game, she learns that Jago is her long lost brother.
Happens twice to Dave Strider in Homestuck. He used to hit on Rose (ironically) before learning that they were brother and sister. In Act 6, he and Rose see their mother in a dream, resulting in the following conversation later:
ROSE: You're wondering why I didn't tell you?
ROSE: You're specifically wondering why I wasn't forthcoming with an answer to your question at the time, "hey who was that choice babe in the pajamas?"
In Chapter Twelve of Evon, Herodotus and Evon discover (to Evon's horror) that Corrin, the magic student she had a fling with in Chapter 4, is the half-sibling of Evon's they were hoping to find. It didn't help that Corrin got over his shock at warp speed and decided that silly little things like societal taboos don't apply to them because Screw the Rules, I Have Supernatural Powers!.
In Arthur, King of Time and Space, Arthur sleeps with Morgan (in the space arc) and Morguase (in the baseline arc) without realising the relationship. (Or vice versa - when Merlin finds out about Morgan he specifically notes that she should have been able to figure out who the mother of the rightful king must have been, but apparently she didn't. And Morguase didn't either, because if she had she'd have realised that her spell wouldn't work properly.)
In Comix from the Underground, the Comix Scrutinizer reads Kit n Kay Boodle and is surprised to learn that the main characters, who constantly have sex throughout the comic, are not married, but have the same last name. Even after joking that the characters might be brother and sister.
A half-example happens in Dragon Ball Z Abridged, that is possibly an Homage to Back to the Future. Trunks travels back in time but keeps quiet about being a time traveler so he doesn't change the past. Then his future mother Bulma starts hitting on him:
Bulma: So hey, like just gonna throw this out there. You're really cute.
Trunks: Well, you know, my mom always said I was a cute kid.
Bulma: Oh a momma's boy huh? I'll be your mommy. *winks suggestively*
On The Cleveland Show, Roberta wears a fat suit using a fake name to see if she can get by without he looks, and sure enough the only person who treats her kindly is her stepbrother, Junior. Unfortunately, he soon develops a crush on her, and she has to pretend to move away to avoid breaking his heart or revealing the truth.
In Family Guy, hit on (no Pun intended) in a later Halloween episode. Meg and her friends actually get invited to a costume party, and she goes dressed as a Slutty Cat, complete with mask covering 75% of her face. One game of spin the bottle/seven minutes of heaven later, she's escorted to the closet with someone in an all-concealing Optimus Prime costume. When they're in there too long, the door is opened to reveal it being Chris.. While they're half-naked on the ground making out. They didn't take it well, and lines like "We did so much!" imply there was more than just lips involved, though they seemed to have gotten over it by the end of the episode, apparently realizing that (given who they are) they were lucky to hook up with anyone at all that night.
"Roswell that Ends Well" in Futurama. While stuck in 1947, Fry accidentally gets his grandfather killed and figures that since he still exists, the attractive woman he thought was his grandmother couldn't possibly be. After he does "the nasty in the past-y", Professor Farnsworth explains to Fry that not only is she his grandmother after all, Fry's grandfather is Fry himself.
Averted on King of the Hill: Joseph is getting closer to a girl who is actually his half-sister. To prevent them from actually doing anything, Peggy invokesFriend Versus Lover with Bobby, making sure he's always hanging around them so that they can't actually do anything together.
An episode of The Simpsons has Mayor Quimby telling his date at a wedding to say she's his niece if anyone asks. She responds "I am your niece, Uncle Joe!", and Quimby mutters "Dear God, I'm an abomination!"
A technical version appears in Teenage Mutant Ninja Turtles (2012). Leonardo and Karai have a very Dating Catwoman relationship. It's later revealed that Karai is Splinter's daughter, making Leo her half-brother by adoption (unless fan-theories of the mutagen making the turtles Splinter's biological sons are correct)
There was an incident, somewhere in this world, where a doctor (secretly) used his own sperm for all his artificial inseminations, meaning that a community now has many couples who might well be half siblings.
A Minnesota School hosts a pep assembly. Part of it is having various athletes receiving blindfolded special kisses from a secret admirer... Their parents.
This is a classic pep rally "gag" of long tradition, though normally it is much more innocent. Blindfolded football players are told they must guess which cheerleader kissed them (usually just a chaste peck on the cheek)...lift blindfold — surprise! It was actually Mom - ha-ha. (Embarassment and teasing ensue.) However, in the incident referenced above, parents were practicallly making out with their kids.
Happened to no less than Giacomo Casanova. According to his autobiography, he was on the verge of marrying a beautiful seventeen-year-old girl when recognized the girl's mother as a former lover...from seventeen years ago. (In real life he broke off the engagement immediately; in his liberally fictionalized autobiography, however, he cheerfully admits to an incestuous threesome.)
There was a case in the UK where twins who were separated at birth got married without realizing they were related. They had the marriage annulled after they discovered the truth.
Somewhere in Britain there allegedly is a gravestone raised in the late 17th century to a woman who was the buyer's "mother, sister, mistress and wife". Apparently the deceased was impregnated as a young woman by her own father; the baby boy was sent to another village and came back a man, started an affair with an older woman he met there and later married her, and only then did she recognise a birthmark on his body...