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.
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.
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).
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.
The Anna Pigeon mystery A Superior Death includes a consensual brother-sister couple who have had themselves sterilised so there will be no children. Anna pities them more than condemning them.
A rare brother/brother example is part of the final twist in the novel As Meat Loves Salt. It is not portrayed romantically.
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 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.
The Beebo Brinker Chronicles: The book 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.
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 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.
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.
Book of Daniel by E.L. Doctorow ends up here. Brother and sister Daniel and Susan eventually consummate their relationship.
Brat Farrar heads in this direction. He's not actually her brother; she just thinks he is.
Ella from Bright Lines is in love with her adopted sister Charu. Charu is biologically her cousin, but they were raised as siblings after Ella's parents died.
In Bring Up the Bodies, Thomas Cromwell is told by Lady Rochford, Anne Boleyn's sister-in-law, that Anne and George Boleyn kiss. When he says that's not unnatural she specifies that it's with tongues. Cromwell is at first skeptical, but works up a rationalization that he can present to the court—they were raised apart, met as adults, a child of the union wouldn't be an obvious bastard because it would look like a Boleyn—and it works for the purpose of getting both of them convicted.
A Brother's Price: Little Eldie Porter's parents were brother and sister, though Not Blood Siblings—though it's still considered incest. The other characters don't realizse they weren't blood siblings at first, and genetically they wereKissing 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.
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.
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 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 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.
Twins and consorts Gerridon and Jamethiel were responsible for a great betrayal in the distant past. Their twincest and betrayal were unrelated, but all the same, ever since then twincest has had some mixed connotations in the culture.
All the way through the first book, Jame and and Implied Love Interest Bane seem on the verge of Slap-Slap-Kiss. They never quite do. At the end of the first book, they are revealed to be half-siblings, which gives it shades of Surprise Incest. It's all very subtexty and unclear. Much later, in the eighth book this finally gets confirmed, with Jame naming him as one of the people she's been attracted to.
Jame (the protagonist) and Torisen (the deuteragonist) are twins. The mutual attraction has so far led to The Big Damn Kiss, and an Erotic Dream. There's no real relationship between them so far, it's heading in that if for no other reason than that, as the author stated in an interview, she simply does not find any other male character in the books as interesting as Torisen.
Timmon's parents, Pereden and Distan, were half-siblings and consorts. They seem to have been pretty happy together.
Timmon: I may have older half-brothers, you see, but my parents were half-siblings, which counts for a lot in our house. [grins] Are you shocked?
This example is very different than the first three: It's homosexual, non-consensual, and not socially condoned: Greshan raped his little brother Ganth when they were young. Ganth told their father, who didn't believe him. It mentally and emotionally scarred Ganth for life.
Tiggeri had his eye on his half-sister Must ever since she was little, and he eventually raped her. This example is different than the others because Must is a half-Kendar bastard, and in their world, Highborn men either raping, or having coercive Unequal Pairing relationships with, Kendar women just because they can is far from uncommon. The scenario with Tiggeri and Must is also something of a deconstruction of Unlucky Childhood Friend.
The Chronicles of Magravandias: Twins Valraven and Pharinet Palindrake fall in love and being an incestuous and adulterous affair. They almost conceive a child, but Pharinet miscarries. That doesn't stop the relationship from destroying their family and their spouses.
The Cornelius Chronicles: Jerry Cornelius has a relationship with his sister. His brother Frank tries to as well. And in a later story, Jerry is referred to as "the last of the truly innocent motherfuckers."
Darkover: 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 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.
Deryni: In 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 Ian McDonald'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.
Twice in 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 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 Divine Comedy: She may try to downplay it, but by having an affair with her husband's brother, Francesca not only damned herself for adultery, but also for incest. Sure, they're in-laws, but if we can take anything from Dante dismissively remembering them as "the two kindred," it's that he didn't think Hell would see the distinction.
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 as of the ending of Petals on the Wind 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 The TV movie of Seeds of Yesterday (the final book in the series), takes this into the next generation, with Cathy's son Bart and adopted sister Cindy getting married after spending the majority of the film at each other's throats.
Many of the works subsequently published under the V.C. Andrews name 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), the 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.
Downtiming The Night Side 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 Jack Chalker book) with the only other people available.
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.
Mentioned as a possibility several times in Dune, usually with regards to Paul the Kwisatz Haderach. The Bene Gesserit have a plan to have Paul marry and have children with his sister Alia in order to preserve his genes, but Paul saw that coming (being able to see the future is useful in that regard) and their manipulations failed. Later, Paul's twin children Leto II and Ghanima briefly mention the possibility when discussing their future plans, but dismiss it immediately. They do end up marrying in order to establish Leto's rule as God-Emperor, but it was a Sexless Marriage (taboos aside, Leto can't have sex due to his transformation; all of Ghanima's children were fathered by her consort).
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 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.
In The Elenium trilogy by David Eddings, 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. It eventually is revealed Lycheas is the product of Arissa breaking another taboo — seducing and having sex with a priest.
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.
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.
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.
Forbidden deconstructs and partly reconstructs this trope. It's about siblings Lochan and Maya, and 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.
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.
Ghoul: Rika and Saxon Hyde provide a pretty Squick-tastic version. It started when they were teenagers, and Saxon was trying to molest their other sister, Debra.
The History of the Galaxy setting has the St. Ivo family, the owners of the Galactic Cybersystems Mega-Corp. The original founder of the company, Erlik St. Ivo, intentionally married his cousin in order to keep the ownership of the company and its secrets in the family. They used genetic engineering to bear a son and a daughter who were not genetic siblings, convincing them to continue the tradition. However, after a few generations, even genetic engineering couldn't fix the problem, culminating in the birth of André St. Ivo, whose sub-par intelligence was insufficient to properly run the company. Deciding to break with tradition, André's parents did not have a second child, instead forcing him to find a wife elsewhere. He married Theia Mitchell, a smart and beautiful woman who effectively ran Galactic Cybersystems in her husband's stead. Jealous, André had her publicly assassinated, making sure she was Only Mostly Dead, and had an Evilutionary Biologist turn her into a compliant sex slave. André later died, and the freed Theia ends up accidentally killing her eldest son Aramant, mistaking him for André (thanks to this trope, they look nearly identical). Theia dies from genetic complications some years later, while her other son John Mitchell St. Ivo (who looks like Erlik for the same reason) is mortally wounded and forced to undergo Brain Uploading into an android body (coincidentally built in the image of Erlik). Many years later, John ends up cloning himself a new body from a genetic sample taken prior to his body's death and becomes able to continue the St. Ivo line, although the corporation is long-gone by that point.
The novel The Holy Sinner by German author ThomasMann features this, as well as parental incest. It is a retelling of the 12th-century epic poem Gregorius or The Good Sinner by Hartmann von Aue, which in turn is based on a medieval French version of the story.
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 Vexvelts harmonious social relations, it is argued, is precisely Vexvelts social acceptance, indeed encouragement, of incest.
I Favor The Villainess has Lambert and Lene Orso, brother and sister, respectively. The portrayal is decidedly unsympathetic when he abuses his position and power to stage a working class revolt, a mass murder attempt of the aristocrats and royalty of the kingdom, and using his demon-sealing skills to unleash an extremely powerful one to do his dirty business. After the end of the arc, their family is disgraced and exiled, though it's implied they're now living well together in another country thanks to the protagonist's intervention.
In Immortals After Dark, the antagonist of one book is screwing his half sister... because she somewhat resembles the half sister he really wants to be screwing.
In Death: Piper and Rudy Hoffman in Holiday In Death are fraternal twins and longtime lovers; Piper even had herself surgically sterilized, with Rudy's encouragement. Eve, herself a victim of incestuous rape by her father, is highly squicked out by this, and there are some indications that the relationship has taken a toll on Piper mentally (though it's not entirely clear how much of this is projection on Eve's part).
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.
The narrator in Invisible by Paul Auster describes a particular incident in his childhood extremelyexplicitly.
Played without angst or tragedy in Jacob's Ladder Trilogy. Since it happens on a generation space ship that's been out and about for almost a thousand years, the "ruling" family wants to keep their blood "pure", as the people on the ship are devided into "Exalt" and "Mean". Thanks to advanced technology and genetic engineering, marriage and children can happen without the genetic downsides. The various sibling relationships are portrayed as diverse as other kinds of relationships and tend to go the True Love route.
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 Legend of Drizzt: 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...
Lensman: The end of the 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.
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.
The 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 Machineries of Empire has Shuos Mikodez and his body double/younger brother Istradez, who have been lovers for decades by the time they appear in the story. Their parents don't mind, and are actually quite happy that their two most hyperactive children have something to occupy themselves with.
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.)
Middlesex is a Generational Saga that begins with the story of the incestuous relationship between the protagonist's grandparents Desdemona and Lefty. They were starting to like each other, and Lefying made a joking wedding proposial to Desdemona. Then on a night they were pretty sure they were going to die, Desdemona had the nerve to accept it. They didn't die that night, so then they emigrated to America and had two kids.
In Mirror, Mirror, a retelling of Snow White, has Lucrezia Borgia not only sleep with her brother, but also her father, and the son she conceived with said brother.
In Agatha Christie's Miss Marple story 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.
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 heroine of Daniel Defoe's Moll Flanders finally discovers her long-lost mother. Unfortunately, Moll's mother is also... her mother-in-law. Oops.
The Monk: Ambrosio and his younger half-sister Antonia, though neither of them is aware of their blood relation. Ambrosio only finds out after he's raped and killed her.
In 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.
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.
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.
One Hundred Years of Solitude: The first two propagators of the Buendía 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.
Present in a benign form in regards to Ben in An Ordinary Sex Life. Adrienne, on the other hand...
Griffins Quri and Jin in The Orphan's Tales. Justified, since this the only way their species will survive.
The Other Boleyn Girl has several non-platonic kisses 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, their sister Mary—who he also kisses—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.
It's implied that Depraved Bisexual Jonathan Randall lusts after his younger brother Alexander.
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.
Pin features a complex relationship for Leon and Ursula. While growing up they had no personal space and often appear naked, cuddle and talk about their sexual encounters. As it is later revealed when they have a threesome with Pin (who is an anatomically correct medical dummy) it is mentioned they've done this many times before. In the end with Leon now paralyzed thinking he is Pin, it is implied the now quite mad herself Ursula uses her brother as a living sex toy when she waltzed into his room fully naked all the time.
Prophet's House quartet: 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.
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 penitent 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 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."
The Saga of the Jomsvikings: Knut, the founder of the Danish royal house of Knytlings, is the result of an incestuous affair a Saxon jarl called Arnfinn had with his sister. The parents keep the boy a secret and abandon him in a forest so he will be found by King Gorm of Denmark.
The jarl had a beautiful sister, and he was fonder of her than he should have been and begot a child with her.
In Gladys Mitchell's The Saltmarsh Murders (1932) features a... tightly wound family. 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. And Ada and her half-sister also had an incestuous lesbian relationship when they were younger.
Sharp Objects: Amma clearly has a thing for her older half-sister Camille. She passes a pill of Ecstasy to her tongue to tongue and later crawls into bed with her wearing only panties. Camille's reaction to this is confused more than repelled.
Smoke and Shadows: Siblings Cassie and Stephen engaged in sexual activities—quite often, if Stephen's "there was lots of sinning" is anything to go by. Unfortunately for them, their dad caught them in the act and then got possessed by a malevolent house presence, causing him to violently murder them both with an axe.
The reigning House Targaryen in made a proud tradition of breeding brother to sister for generations.note Failing that, there were uncle-niece, aunt-nephew, and cousin marriages, as well as occasional marriages to other nobles. 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 that they now tend to produce either great heroes or a complete psychopaths, making the Targaryens a sort of on-again-off-again example of Royally Screwed Up. One of their kings, Aegon V, actually wanted to stop this practice, but two of his children decided to marry without his permission. Their children were in love with others, but were forced to marry because of a prophecy.
Cersei and Jaime Lannister, as revealed in the first book, also known asthe Lannincest. It's a major plot point because 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. This pairing is an unusual case of this trope as Cersei and Jaime are actually one of the few genuinely loving relationships seen in the series... at least at first. It's plain to see that Jaime truly loves Cersei and, even when other women jump into his bed, proudly remains loyal to his sister. Cersei is rather less monogamous, and still carries a torch for Rhaegar Targaryen and sleeps around with other men... but mostly because sex is one of the only weapons she has at her disposal as a woman in her society. The other people she sleeps with are either to reach political ends, and occasionally out of loneliness or wanting for feel powerful. She still flat-out says, "It had never been any good with anyone but Jaime." Eventually though, the war changes the twins so much that, once reunited, they come to resent the other.
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 repulsed and embarrassed when he found out.
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 the later book 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.
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.
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 Thirteenth Tale: 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.
Dianora and Baerd were involved like this in their backstories. To Dianora, because Baerd is her brother, he's family—he's home. Her love for him is symbolic of, or at least intertwined with, her love of her homeland. And love of their homeland is perhaps the biggest driving force in the story.
In-universe incest is referred to as the "sin of the gods" because two of their three gods, a brother and sister, were the parents of the third.
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.
Sibling criminals Stanley and Lucinda Clovis in Lawrence Sanders' The Timothy Files. To add to the squick, Stanley is married with two kids.
In War and Peace, there's a rumor that Anatole and Helène Kuragin/a have an incestuous relationship; their interactions lead most readers to come away believing these rumors, and many adaptations make it explicit. Given that the two are willing to sleep with anything that has a pulse, it's extremely possible.
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, trying to keep it consistent with the family trees on the official site (which weren't by an Erin, not her choice to be put online, and not canon - but it was, after all, the official site, so she didn't want it to be 100% wrong). 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. When that fact was pointed out to her, she decided that it would remain canon since it can actually happen with cats (and besides Graystripe's parents aren't actually mentioned in the books, so it's not like young readers would happen across that - it's only canon through Word of God.) Patchpelt being Graystripe's father was eventually retconned out, however.
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.}
Thomas Mann's Wälsungenblut (Wälsung blood) in which a brother and sister have sex after watching a performance of Rheingold
The Wayfarer Redemption features the Icarii, a race of winged humans, for whom anything goes any time after the start of puberty, so long as it's consensual and isn't between siblings or between parent and offspring (both of which are absolutely forbidden, to the point where even villains will describe it as "Unclean"). Unfortunately, the ruling SunSoar family has a curse - like calls to like for them, and siblings Caelum and RiverStar discover the hard way just how badly things can turn out. He kills her when she tells him she's pregnant with his child and frames her twin brother for the murder.
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.
Maria McCann's The Wilding, involves a Surprise Incest twist between a couple who are at least half-siblings and possibly full.
Mignon of Wilhelm Meister's Apprenticeship is the product of BrotherSister 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.
In the 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).
Clark Ashton Smith's Vathek story The Story of the Princess Zulkais and the Prince Kalilah, has the titular prince and princess. Born to an Emir of muslim Egypt whose latest wife dies in childbirth, these twin aristocrats had been treated with unholy sorcery at the Emir's request to give them great talent. Between these treatments, growing up isolated from their older half-sisters and being twins, the two develop a more than filial love for each other and join with the djinn lord Eblis so that they can have their forbidden love.
Lilith's Brood has a variant with the Cthulhumanoid Oankali: parental units comprise a male and female, who are usually brother and sister, and an "ooloi" third sex from a different family. Justified (from a biological perspective, at least) by their innate ability to manipulate DNA; males and females seek out and concentrate desirable genetic traits, while ooloi combine them into Designer Babies without the usual disadvantages of inbreeding.