In the novel Frankenstein, Victor and Elizabeth court and marry. They're not biologically related, but they were definitely raised as brother and sister.
Forbidden by Tabitha Suzuma is all about the growing unstoppable love between Maya and her brother Lochan. Their relationship is shown in a sympathetic light as they did not have a normal childhood. They lost their father early and having a callous mother made them look after their younger siblings. They do commit incest towards the end of the book. A point is made several times that neither of them feels anything for their other siblings beyond the norm.
Happens in David Wingrove's Chung Kuo series with Ben Shepherd and his sister.
Present in a benign form in regards to Ben in An Ordinary Sex Life. Adrienne, on the other hand...
The Dollanganger series by V. C. Andrews. The first two books, famously starting with Flowers in the Attic, focus on two siblings Cathy and Chris who, along with their younger twin brother and sister, are locked in an attic by their grandmother and escape after the male twin dies of poisoned donuts. Turns out the reason their Grandmother Olivia locked them up is because their Mother and Father (who died in a car accident making it necessary for them to move to their Grandparent's mansion) were half-uncle and niece and were disowned because they were caught having sex and then ran away. The big issue is that Cathy and her brother Chris have an incestuous relationship as teenagers that Cathy tries to avoid but, among other things, apparently Rape Is Love, and they end up living as husband and wife in an open secret type of relationship. The big surprise comes in the fifth book and prequel when you find out Cathy's parents were not half-uncle and niece...they were actually half-brother and sister AS WELL as half-uncle and niece due to Cathy's grandfather Malcolm raping his stepmother Alicia. They never knew as Alicia and her second husband died and Malcolm, Olivia, and Olivia's cousin John Amos NEVER told anyone. All of this leads to (Grandmother) Olivia becoming the cold cruel person she is in the later books in order to hide this from the world
Many of the author's works are known for this trope, including the Casteel series (Heaven and her uncle Troy, Heaven's daughter Annie and her (apparent) half-brother Luke), the Cutler series (Dawn and Jimmy, Dawn's actual blood brother Philip who remains creepily obsessed with her even after he learns they're related and later goes after her daughter Christie), Landry series (Ruby marries her secret half-brother Paul so he can pose as the father of her illegitimate child and later has a one-night stand with him) and Broken Flower.
In Jean M. Auel's Earth's Children series, Jondalar's half-sister is madly in love with him. 'Course, since these characters don't understand where babies come from and Jondalar and Joplaya's common parent is their father, they are believed to be something closer to cousins than siblings. Still squicky though.
The romance between Sam and Astrid in the Gone series is absolutely hilarious if you've seen this quote from his wife, K. A. Applegate:
I don't usually use real people. Michael used the kids for GONE and then ended up horrified when he realized he'd hooked up our "son" and "daughter" romantically.
The Prophet's House quartet has this in spades with the incestuous House Rassianus, most notably in the Backstory between Sepirahkt and Areina, but later Alisayne and Lysander get in on the action.
The Anna Pigeon mystery A Superior Death by Nevada Barr includes a consensual brother-sister couple (who have had themselves sterilised so there will be no children). Anna pities them more than condemning them.
In Marion Zimmer Bradley's The Mists of Avalon, Morgaine and Arthur have sex and conceive a child as part of a religious ritual without either of them being aware of the other's identity. When they find out Arthur is appalled, but Morgaine maintains that since they were possessed by the God and Goddess at the time it wasn't really them.
However, she does feel guilty about the enthusiastic non-possessed morning sex they had after waking up the next morning, and immediately before recognizing each other.
Kind of further confused by the fact that Morgaine is capable of just leaving it behind her (more or less) while Arthur is still in love with her.
Even more confused because Viviane, who arranged the whole thing, apparently thinks this is no biggie and Morgaine should take the opportunity to become Arthur's consort and influence his reign in Avalon's favor. Viviane seems to regard [half]brother/sister incest as a specifically Christian taboo (it isn't), an influence that Morgaine should be above as a priestess of the Goddess.
The titular characters of Kelly Braffet's Josie and Jack have an increasingly sexualized relationship, but it's deliberately unclear if they ever consummated it.
The love between Heathcliff and Catherine Earnshaw in Wuthering Heights has overtones of this, but don't worry — he's adopted.
Because of the way Mr. Earnshaw randomly brings Heathcliff home one day, some people do theorize that he's his son by another woman, which would make Heathcliff and Catherine half-siblings.
In Speaker for the Dead, sequel to Ender’s Game, Miro and Ouanda are outed as half-siblings by Ender's Speaking, due to Miro's mother's adulterous relationship with Ouanda's father. Later they both remark that it's lucky that, due to strong Catholicism, they never had sex.
Although one of them points out later that if further tragedy hadn't struck right after that revelation, they might have made some attempt to continue the relationship - but what happened next would have drastically changed things for them anyway, and therefore made it easy for one of them to break it off.
Actually, Miro wasn't so big a Catholic that he would abstain before marriage. Ouanda was the devout one. Miro reached the same conclusion through rational means, reasoning that only a large community can afford to have people break religious taboos. Their community is very small.
In murder mystery The Demoniacs by John Dickson Carr, the early-eighteenth-century detective has come to suspect that his girlfriend is his half-sister. She isn't. If she is, he wants to find and destroy the evidence so he'll be able to marry her anyway. When he admits that to her, after finding out she isn't related, she's pleased rather than squicked; she plainly feels it's romantic.
Downtiming The Night Side by Jack Chalker presents this as the inevitable consequence of growing up marooned on a small island with only your parents and siblings for company. It's only natural for the kids to want to do what mom and dad do (a lot, since it is a Chalker book) with the only other people available.
In Agatha Christie's Sleeping Murder, the killer was the victim's brother, who had tried to prevent her from forming any romantic attachments, and killed her after her marriage to another man.
In Cassandra Clare's ''City of Bones'' a fortune teller tells Jace that he will "Love the wrong person". He falls in love with Clary, who turns out to be his sister.
Subverted. They end up not related. Jace is the son of his supposed Father's second second-in-command. And Clary ends up with Jace, though only after meeting and kissing a boy she finds out is her REAL brother. Oops.
In City Of Lost SoulsSebastian/Johnathan is shown to actually want Clary, backing her up against a wall and forcefully kissing her, while going on about how they can be evil and rule the world together.
The heroine of Daniel Defoe's Moll Flanders finally discovers her long-lost mother. Unfortunately, Moll's mother is also...her mother-in-law. Oops.
In the David EddingsElenium trilogy, King Aldreas has an incestuous relationship with his sister, Princess Arissa; there is a lingering question, for part of the story, whether he was the father of her son Lycheas.
Middlesex by Jeffrey Eugenides begins with a portrayal of the incestuous relationship between the protagonist's grandparents. They get married, emigrate to America, and pop-out a kid.
In The Sound and the Fury, it is implied that Quentin is in love with his sister, and even offers to have sex with her so that he can claim that her out-of-wedlock child is his, shifting the blame to him and preventing her from dishonoring the family (exactly how this would have helped is dubious). She says okay, but he chickens out. He then tries to tell his father that he has "committed incest" with his sister, who doesn't believe him. To top it off, Quentin later commits suicide, and his sister ends up naming her daughter after him.
It's also a major plot point in Absalom, Absalom, in which Quentin is told the story of Henry Sutpen, who protected his sister from the advances of her half-brother (although he, in the end, manages to convince himself he's okay with the incest, just not the fact that her half-brother has a fraction of black ancestry). This is but part of the huge amount of emotional baggage Quentin gets dumped on him.
Twice in Katharine Kerr's Deverry series; the first time in Daggerspell, as part of the ancient tragedy that drives the first four books, involving a despairing heroine dumped by her one true love, the second time as one of the many acts demonstrating the corrupt nature of the villainess.
In Dean Koontz' The Bad Place, the villain, generally known as "Candy", and his brother, co-protagonist Frank, are two of the offspring of hermaphroditic Roselle (fully reproductively functional as either sex), who was herself the product of brother-on-sister rape.
The villain's sisters, Violet and Verbina, have an implied attraction for each other and a rather blatant one for Candy. Considering that this eventually gets them killed, the latter is definitely not played for eroticism.
Katherine Kurtz's Camber of Culdi. The last king of the Festillic dynasty, Imre I, has an incestuous relationship with his sister Ariella, which produces the line of Festillic pretenders who for generations proceed to plague the Haldane dynasty.
In Sharon Maas's Of Marriageable Age, Saroj finds out Nat is her half-brother, only after she's carrying his child. Of course it turns out she's adopted, in the end..
In Gabriel García Márquez' One Hundred Years of Solitude, the first two propagators of the Buendia family are cousins, and a major theme throughout is the prevention of the family tree from getting too tangled, for fear of "bearing iguanas". There is much Squick. In fact, at the very end, one character becomes the lover of his aunt, thinking she's his long-lost sister.
The reigning House Targaryen in A Song of Ice and Fire made a proud tradition of breeding brother to sister for generations (though with occasional marriages to other nobles, especially when there was no sibling of opposite sex or for political reasons). Though most Targaryens have a penchant to be silver-haired and violet-eyed, their insistence on keeping the bloodline "pure" for three hundred years or so means also that they now tend to produce either a great hero or a complete psychopath, making the Targaryens a sort of on-again-off-again example of Royally Screwed Up.
Likewise, Cersei and Jaime Lannister, as revealed in the first book. Cersei's children, including King Joffrey, are not the offspring of her husband Robert Baratheon, but of her Half Identical Twin Jaime. If this were ever proven, the kids would be publicly slaughtered as abominations. Mom's not cool with that.
And then there's the scene with Theon Greyjoy and his sister Asha, where she gave him a deep and rather passionate kiss, and felt him up. In this case, it was a Secret Test of Character: Asha recognized her brother, even if the reverse wasn't true. Plus, Theon was pretty embarrassed when he found out.
Note that these are only the explicit examples; ASOIAF also has a fairly large entry on the Incest Subtext page.
The Night Angel Trilogy has Terah Graesin and her brother Luc. Terah Graesin has some Cersei-ish vibe. She eventually sends their younger sister Natassa to certain and horrible death because the poor thing found out.
Cormac McCarthy's novel Outer Dark opens with a woman giving birth to her brother's baby, and things — as they often do in McCarthy's worlds — quickly go downhill after that.
In Gladys Mitchell's The Saltmarsh Murders (1932), the murderer blackmails the Lowrys into helping in covering up some important evidence on the strength of the knowledge that they are brother and sister rather than husband and wife.
Vladimir Nabokov's Ada, or Ardor: A Family Chronicle chronicles an incestuous family with a focus on a specific brother-sister pair, who end up semi-happily married by the end of the book.
That does not begin to do justice to how tightly wound that family is. Van and Ada start off thinking they're cousins—and second cousins, as while their mothers are sisters, their fathers are cousins. They later find out that they're both the product of an affair between the man Van knew as his father and the woman Ada knew as her mother. The two parental pairs were also second cousins. Oh, right, and Ada's sister (or half-sister, as it ended up being) also had a thing for Van and is Driven to Suicide when it is unrequited.
Don't forget that Ada and her half-sister also had an incestuous lesbian relationship when they were younger.
Mervyn Peake's Gormenghast: Titus/Fuchsia is implied, but not explicit.
In Pyramids, Ptraci's father was Teppicymon XXVII, so she is Teppic's half-sister. This is revealed to the reader midway through the book, but the main characters remain unaware of it until near the end. They show UST for each other during their escape from Djelibeybi. After they meet up again back in Djelibeybi and find out that they are related, Ptraci asks "That doesn't make any difference, does it?", to which Teppic replies "I think it does, really."
In the Drizzt Do'Urden novels, Vierna was quite taken with her little brother, but he refused her. That incest is implied to be seen as kinky at best even among dark elves — who, among other things, have sex with demons and engage in public orgies at graduation ceremonies — it kind of drives home that even Vierna, probably the most well-adjusted of the Do'Urden women, is more than a little twisted.
Vierna seemed to think she was doing him a favor; assuming Drizzt was too embarrassed to engage in the aforementioned public orgy, she was offering to help him fulfill his "graduation ceremony" in a private setting with someone he knew, rather than in public with a stranger.
Vierna was warned against such behavior by her mother, so, apparently, the drow DO look upon it negatively. And she warned her about it the night Drizzt was born...
The Thirteenth Tale has Charlie's obsession of sorts with his younger sister Isabelle. Then again, with a family like theirs, it's hard to turn out in any way normal children. It's implied that Charlie is the father of her children.
The Faerie Queene features intercourse between twins in the womb. The giants Argante and Ollyphant, themselves the product of incest, mingle "in fleshly lust" before birth and emerge clasped in the act.
Mignon of Wilhelm Meister's Apprenticeship is the product of Brother-Sister Incest. Her parents did not find out that they were siblings until after they had a daughter and the discovery lead to Mignon's mother's suicide and her father's madness.
The end of the Lensman series strongly implies that the Kinnison kids will do this to produce a new race of superbeings. E. E. “Doc” Smith implied on several occasions that this idea kept him from doing a sequel, as back then such ideas were strictly taboo— if he wrote that into a story, he'd never find a legitimate science fiction publisher who'd accept it. At least here, the Arisians' having bred out all the nasty recessives throughout human history means there won't be any horrible genetic consequences.
Steve and Roz Brickman from Patrick Tilley's excellent Amtrak Wars series had a covert sexual relationship while they were younger. However, given the extremely weird nature of the Trackers, it's arguable as to whether or not Roz and Steve are related biologically (they are emotionally at the very least).
Túrin is based on the character of Kullervo from The Kalevala, who has a similar unknowingly incestuous relationship with his sister.
The not very famous book Malika from french author Valérie Valère is all about the incestous love between a teenaged boy and his little sister Malika. The siblings live on their own in Paris, and have an Absent Father. At one point they confess their feelings to each other and start a relationship. (And have a sexual encounter, and a really, really tragic end.)
Randall Garrett's first Lord Darcy mystery, "The Eyes Have It", had this as its motive for homicide, as a noblewoman had killed her obsessed brother to stop him from raping her.
The Other Boleyn Girlby Phillippa Gregory has at least one non-platonic kiss between Anne Boleyn and her brother George (who in real life as in the book was accused of adultery with her and was also beheaded). Their "closeness" probably stems from the fact that George is the only person Anne can really trust in her precarious position. The narrator, Mary, and her husband speculate that Anne may have convinced George to sleep with her once she was queen so that she could have a son, and that one or more of her miscarried pregnancies may have been incestuous.
George actually kisses both Anne and Mary. Several times. Non-platonically.
In Lord Byron's Manfred the title character strongly implies that his "nameless" crime was incest with his beloved Astarte. His Cain focuses on the hero's love for his sister/wife and his incredulity that though Jehovah has tolerated incest among mankind's second generation, his own children are forbidden to marry each other. Originally the protagonists in "The Bride of Abydis" were a brother and sister in love but were changed to first cousins in the final version.
Edgar Allan Poe's "Fall of the House of Usher". The obsession of Roderrick for his sister Madeleine hints clearly at incest.
It explicitly states that their family tree does not branch.
The Egyptian, as it involves Ancient Egyptian royalty obviously includes this as a matter of course. Mostly in the character of Baketamon who considers her brother as the only man fit to marry her.
Francesca Lia Block's novel Wasteland explores how a relationship like this could even develop. Older brother Lex and sister Marina appear to be soulmates (their attraction apparently began as small children) and they are written as extremly sympathetic. When the relationship is finally consummated, the guilt drives Lex to suicide. And then it turns out he's adopted.}
In P.C. Hodgell's Chronicles of the Kencyrath, there are two sets of brother-sister couples: Gerridon and Jamethiel, who were responsible for a great betrayal in the distant past, and their children fraternal twins Jame and Torisen. The mutual attraction between the latter two has so far only been strongly implied, but Hodgell seems to be heading for depicting a romance if for no other reason than that, as she stated in an interview, she simply does not find any other male character in the books as interesting as Torisen.
Nightside: Suzie Shooter was coerced into a sexual relationship by her older brother as a young teen, got pregnant, and was forced to have an abortion by their parents, who blamed her for "leading him on". She shot her brother, pissed on his corpse, and ran away to the Nightside.
In Stormqueen!, a father finds himself with no living male heir, a foster son (20-year-old son of his dead mistress from her marriage) he loves, and the only heir of his body existing is his preteen daughter. Oh yeah, and he hates his brother. What to do to pass on the succession? Marry off the half-siblings to each other. Yes, he intends them to make a baby. Even worse, the two of them are eventually willing to go along with that. It's only stopped when Dorilys kills her brother after she finds out he knocked up his girlfriend.
In The Dark Volume by Gordon Dahlquist, it's revealed toward the end that Francis Xonck is the father of one of his sister Charlotte's children. It's obliquely hinted at in that the girl's name is Francesca.
In Piers Anthony's Bio of a Space Tyrant series, the titular Tyrant may have had sex with his sister. The books are presented as Fictional Documents by the Tyrant's biographer, and she hedges herself by saying that she doesn't know for certain because neither of them would talk to her about it — which does not stop her from engaging in lurid speculation about it, particularly in The Iron Maiden.
A Breath of Snow and Ashes has Malva and her brother. He not only gets her pregnant but, tragically, kills her when she refuses to go along with his plan to pretend that it's actually Jamie's child in order to coerce money out of the Frasers. Had she gone along with the plan, he informs Claire, they would have moved to another state, where no one would know them - or that they were related. He also implies, while he's blurting out secrets, that he started having sex with Malva when she was quite young. There are hints, throughout the scenes involving the two of them together, that their relationship is a bit more than just brother and sister, but it's not stated explicitly until well after Malva is dead. Claire suspects, though, that their father may have known or at least had strong suspicions.
In Guy Gavriel Kay's Tigana, Dianora and Baerd were involved like this in their backstories. In-universe it's referred to as the "sin of the gods" because two of their three gods, a brother and sister, were the parents of the third.
The novel The Holy Sinner by German author Thomas Mann features this, as well as parental incest.
Sibling criminals Stanley and Lucinda Clovis in Lawrence Sanders' The Timothy Files. To add to the squick, Stanley is married with two kids.
The Reveal of "The Lighthouse Keeper's Daughter" is that Alastair lost his grip on reality and is implied to have committed suicide because he was in love with his sister, Squid.
A major theme of Helen Dunmore's A Spell of Winter is the incestuous relationship between siblings Cathy and Rob. Cathy eventually breaks off the relationship after she gets pregnant, is blackmailed into aborting the baby, goes crazy from the guilt, and murders her blackmailer.
The narrator in Invisible by Paul Auster describes a particular incident in his childhood extremelyexplicitly.
Ian McEwan's "The Cement Garden", where two older children wind up having sex together in the final scene. It only happens once, but when they get caught by their social worker (who is implied to be atracted to the girl), she teases him by saying "Oh, this has been going on for ages and ages". Sanity did not have a strong grip on any member of that family, even the parents.
In Ian Mc Donald's Desolation Road the eponymous town's dentist has an unspoken and unrequited (she knows about it but is indifferent) love for his sister. When she "cheats" on him with another man he commits suicide.
In "Ares Express" which occurs in the same universe two minor characters are the postmasters of a small town and apparently come from a long line of inbreeding resulting in both of them, especially the brother, being rather dim-witted. Whether they indulge in the same pastime is left vague.
In L. Jagi Lamplighter's Prospero's Daughter trilogy, an Attempted Rape in the Back Story turns out to have been a half-brother attacking his half-sister, thoroughly Squicking both the victim and the now petitent would-be rapist on top of their already strong reactions to the attack. Furthermore, Logistilla and Titus had children together in hopes that that would persuade Prospero to give the children the Water of Life; they are sick of watching the children die of old age.
In Innocent World by Ami Sakurai, main character Ami, seventeen years old prostitute and Author Avatar, has a long term sexual relationship with her brother. And to add more Squick, he is mentally handicapped. Also, this relationship is portrayed as Ami's only way of feeling close to true and innocent love. Needless to say, the novel seems to be aimed mainly at shocking the reader with the depravity of Japanese teenagers.
In Sky Lee's book Disappearing Moon Cafe, a half-brother and sister (who are unaware that they have a parent in common) have a sexual relationship.
The play Minach by Iva Volankova, the first act is about a sibling pair wherein Brother (they're not named) has been romantically/sexually obsessed with Sister since childhood, to the point where it is implied he killed their parents for trying to stop him sleeping with her, and is angry and bitter at Sister for refusing him. For her part, she realizes this isn't healthy but also seems to think she's been ruined for all other men by it. At one point she breaks down, he starts kissing her and carries her into the bathroom for implied offstage sex.
In Melissa de la Cruz's Blue Bloods series, twins Jack and Mimi Force are actually the angels Abaddon and Azrael, respectively, and are vampires who are 'immortal twins'; that is to say they are bonded forever. This life cycle just happens to have them as blood relatives, which doesn't really seem to stop them - until Schuyler Van Alen (who is so many tropes in and of herself) steps in.
In Mario Puzo's The Family, Lucrezia Borgia loses her virginity to her brother Cesare. Their father, the Pope, who told them to do it, stays in the room to watch. Lucrezia and Cesare are in love for the rest of their lives.
Also dealing with the Borgias (who were historically defamed as a bunch of incestuous murderers), Gregory Maguire's Mirror Mirror, a retelling of Snow White, has Lucrezia not only sleep with her father and brother, she also seduces her son with Cesare.
In Barbara Vine's novel The Chimney Sweeper's Boy, the final twist is that Gerald Candless abandoned his family and changed his name because he unintentionally had sex with his brother in a bathhouse.
In Walter Jon Williams's Angel Station, Beautiful Maria (yes, that is her full name) and Ubu Roy are not genetic siblings, as they were both genetically engineered by their "father" from scratch. This is most evident by Ubu's four arms and Maria's technopathy. However, they have been raised as brother and sister, so their feelings for each other are no different than if they had biological ties. That said, they have no problem screwing each other whenever they feel like it (it gets pretty graphic at times), although this can be justified by the fact that (after their "father's" suicide) it's just the two of them on long voyages and that their "father" used hormones to boost their growth, which made them all the more horny. They don't see a problem with this, probably because they have limited exposure to other people and they don't really advertise this.
Oh, and their ages are 13 and 11, although, thanks to their hormone treatments, they look like adults.
In Mikhail Akhmanov and Christopher Nicholas Gilmore's Captain French, or the Quest for Paradise, the titular protagonist's new wife decides she wants children, but he points out all the problems of having children aboard a ship that spends most of the time traveling from planet to planet with very low gravity aboard. Even if they spend all her pregnancy on a planet, they would still have to raise the baby aboard the ship with no one his or her age around. Additionally, after a certain age, the son or daughter will wish for a partner, and that adds another layer of problems. She reads up on genetic engineering and suggests having opposite-sex twins, whose DNA is slightly altered, so that they're not actually siblings. This would allow them to marry later. He points out that it doesn't matter what genes say; it's being raised as brother and sister that'll make it feel extremely wrong.
The Pearl Of Great Price one of the four 'standard works' is LDS (Mormon) canon and scripture says that the sons and daughters of Adam and Eve began to 'divide two and two in the land ... and they also begat sons and daughters' meaning it's explicitly Mormon doctrine that EACH AND EVERY SINGLE PERSON ON EARTH is a product of sibling incest. While this neatly explains where Cain's wife came from (she was a sister) it ... brings up other issues.
Said issues also exist in the book of Genesis, really. If Adam and Eve's children weren't marrying siblings, who were they marrying, since there was a notable shortage of other choices?
A rare sister/sister example crops up in The Blindness of the Heart (Die Mittagsfrau): Martha and Helene have a sexual relationship for a while; Helene doesn't really seem to dislike it, but she's clearly being taken advantage of by her older sister. Ends when Martha and Leontine enter a full-time relationship.
In Gary Jennings Aztec, Mixtli the protagonist is forced via drugs into having sex with his older sister during the first arc. This later leads to genuine love between the two, at least until a Jerk Ass prince steps in...
The Dresden Files: One of the short stories opens with a brother/sister pair who've been whammied into being passionately in love. Eventually the wrongness in their subconscious got too much and they shot themselves. While having sex.
For Warrior Cats, when the author was asked about parentage of some of the main characters, she said that Graystripe's parents were Patchpelt and Willowpelt. However, she forgot that in Bluestar's Prophecy, we see that Patchpelt and Willowpelt are brother and sister - they're both kits of Adderfang and Swiftbreeze, just in different litters. To be fair, she'd only done the Willowpelt-Patchpelt pairing in the first place to keep things consistent with the family trees on the official site, which weren't by an Erin, weren't Vicky's choice to be put online, and weren't at all canon - most of the Bluestar's Prophecy lineages were based on that tree so that the trees wouldn't be 100% wrong - after all, it's the official site.
Twins Valraven and Pharinet Palindrake fall in love and being an incestuous and adulterous affair in The Chronicles of Magravandias. They almost conceive a child, but Pharinet miscarries. That doesn't stop the relationship from destroying their family and their spouses.
Griffins Quri and Jin in The Orphan's Tales. Justified, since this the only way their species will survive.
Ambrosio and his younger half-sister Antonia in The Monk, though neither of them is aware of their blood relation. Ambrosio only finds out after he's raped and killed her.
“If All Men Were Brothers, Would You Let One Marry Your Sister?” is a science fiction short story by Theodore Sturgeon. It first appeared in Harlan Ellison's anthology "Dangerous Visions" in 1967. It is about an Earthman who manages, with some difficulty, to go to the planet Vexvelt, a paradise which is shunned by the rest of civilised universe for unknown reasons. He finds it a utopian paradise, but then discovers to his shock and horror that incest is actively encouraged there. Indeed, a large part of the reason for Vexvelt’s harmonious social relations, it is argued, is precisely Vexvelt’s social acceptance, indeed encouragement, of incest.
In the short story "Chinandega" by Lucius Shepard, the protagonist's sister becomes his favorite prostitute. They only start this after an admitted liar claims she had been adopted. The truth is never established.
In the novella "Motherland" by Graeme Penman, many of the flashbacks seem to imply that Valery had a sexual relationship with his sister and it is pretty strongly hinted that the habit runs in the family.
The Old Norse Saga of the Jomsvikings claims that Knut the Foundling, the founder of the Danish royal house of Knytlings, was the product of an incestuous affair between a certain Jarl Arnfinn and his own sister.
A rare brother/brother example is part of the final twist in the novel As Meat Loves Salt. It is not portrayed romantically.
The author, Maria Mc Cann, uses the trope again in The Wilding, although this time the twist involves Surprise Incest between a couple who are at least half-siblings and possibly full.
It's found in A Brother's Price that little Eldie Porter's parents were brother and sister, though Not Blood Siblings; genetically they were Kissing Cousins. In this world a man marries every sister in a family, and the sisters themselves are descended from a series of sisters and so on. It's still pretty savagely taboo in this world.
In one of his stories Lazarus Long recounts the time he met a fraternal twin couple who due to the circumstances of their conception had inherited different genes from their parents and as such were no more related to each other than their parents had been. They had several perfectly healthy children.
Also comes up in To Sail Beyond the Sunset, with several mentioned instances of sibling incest among Maureen's children. The only such pair she has an issue with is Donald/Priscilla, because she considers their rather exclusive pairing (especially on Priscilla's side) unhealthy, since a marriage would not be advisable due to the genetic relationship (this part is in the mid-20th century). More casually recreational encounters among her children, she's fine with if they're careful to avoid conceiving.
In the short story "Florville et Courval" by the Marquis de Sade, Florville falls in love with her brother Senneval and bears his child, though he abandons her, taking the child with him. She had been given away secretly by her mother at birth, and they were unaware of their biological relationship.
Ann Bannon's novel The Marriage is about a man and a woman who don't find out that the man is the woman's long-lost older brother until after they've married and conceived a child. In the end, the child is born healthy and the couple decide to stay together without telling anyone else about the incest.
A sister/sister version in the Gaea Trilogy by John Varley. April and August Polo are very much a couple despite being identical twins—which the protagonist comments isn't very surprising given that they (and their seven other sisters) are all clones of their "mother" created after her death and raised in a lab with no outside friends.
A brother and sister in one of the In Death books has been engaged in this long-term. The brother even convinced the sister to get sterilized to prevent conception. Needless to say, this creeps out Dallas, a former Father Daughter Incest victim, a whole lot.
Coira and Hadz in White as Snow are very likely half-siblings, but that's not going to stop Hadz from claiming her as his Persapheh.
In Andrzej Sapkowski's short story The Witcher the titular hero is hired to kill (or disenchant) a strigga - a monster that is the undead offspring of King Foltest and his sister Adda (who died in childbirth).
Age of Fire: After the death of her mate/great-uncle SiDrakkon, Infamina is engaged to her brother (and new Tyr) SiMevolant, with whom it's implied she was already having an affair with. That being said, it doesn't appear as though dragons have too much of a taboo on incest, as the only objection raised to this relationship is it's announced almost immediately after SiDrakkon's death, skipping the mourning period.
After Cain killed Abel, he went into exile away from his family, but he went on to marry some unnamed lady. There being no other families around, this would have to mean one of his sisters came with or after him.
Abraham and Sarah were also half-siblings, and all of Noah's grandchildren would have had to marry at least their first cousins. In fact, Biblical chronology indicates that what we now refer to as incest has to have happened at no less than two points in history or else none of us would be here.
Not only for humans. Aside from bringing seven pairs of a few species designated acceptable for sacrifice (and any domesticated animals the family might have had), the Biblical account puts "every air-breathing animal on the planet" through the same genetic bottleneck.
Adam and Eve, or is that Parental Incest? She was made from him, which might have been something like having him give birth to her.
In the book of Genesis, Eve was made from Adam's rib, so unless God worked some additional magic, they were brother/sister in a cloning sense. The book doesn't tell us where Cain and Seth's wives come from, but Adam and Eve are the only established source, so it's probably supposed to be a common sense conclusion. Since humanity is still young and vital, it's a case of Incest Is Relative until more careful selection is possible or even necessary. But fifteen hundred (and change) years later, the punitive Great Flood leaves only four married couples alive — three of the men are brothers, and the fourth couple are their parents. The following children would have no one to marry but their cousins or siblings. As Genesis also claims that people lived an average of 400 years back in those days, the much shorter lifespans following the Flood can be taken as a result of that tragically narrowed gene pool. This is all consistent with the Bible's theme of various cataclysms as a consequence of man's pride leading to repeated falls.
It says Cain and Seth found wives, and I think the implication is that there were other people created other than Adam and Eve. Maybe they were just first. Arguably, the earlier stories in the Bible were never meant literally, anyway.
Later, Abram and Sarai, who would still later be called Abraham and Sarah. Abram asked Sarai to use this fact so he wouldn't be killed over her. She agreed, and they went down to Egypt. The Pharaoh at the time had this lovely 65-year-old taken to his house, and the Pharaoh and his house were subsequently stricken with great plagues. Upon learning that Abram had not told him the important fact that she was his wife, he told Abram off and sent them away. Fast forward twenty-five years, and they went down to Gerar, still planning the same omission. Abimelech took her, and learned from a vision that she was Abraham's wife, and that Abimelech would die if he didn't give her back. He did, but asked Abraham what could have possessed him to omit such an important detail. He explained his reasoning, but also that she was, in fact, his sister from another mother, but from the same father. He let them live there and gave them stuff, and Abraham prayed and the wombs of Abimelech's house were reopened from being stopped up. (Bet that experience made him wary when second cousins Isaac and Rebekah came into town with the same story...)
Actually, Rebekah was Isaac's first cousin once removed, he having been quite the latecomer to Abraham's family and all of his first cousins being well into their child-bearing years by the time he was born; and yes, Abimelech did figure them out pretty quickly. Like father, like son!
David's son, Amnon, lusted after his virgin half (?) sister, Tamar, so badly that he pretended to be ill so that Tamar would come to his room to feed him. He then raped her over all of her protests (and it being a gigantic no-no in Jewish law). Some try to argue they were Not Blood Siblings.
The interesting part is where she pleads with him to talk to their father about it with the assumption that David would aprove the marriage. Now it's very possible that she was just trying to put him off but it can be assumed that she wouldn't have made the plea if it was completely implausible.
Other than the story of Amnon and Tamar, however, all of these incidents took place before the books of Leviticus and Deuteronomy, which specifically outlawed these practices, were written. The very word "incest" is a term that has been retro-actively applied, coming as it does from Latin, the language of ancient (yet still much younger than Israel) Rome.
Though there are indications that the Law was already known to some extent before Moses wrote it down (e.g., Noah knew which animals were clean before Leviticus 11 was written).
Invoked in Richard Ellis Preston Jr.'s Chronicles Of The Pneumatic Zeppelin; although he was seven when adopted, and Max an equivalent age (for a Half-Human Hybrid), and Sabrina was thirteen, Romulus feels guilty about being attracted to either, because they are adoptive siblings.