In Bored to Death, Jonathan goes looking for his real father and meets and sleeps with a woman named Rose, he discovers later that she is in fact his half sister. He chooses not to tell her and it is implied that they carry on the relationship.
At the climax of the canceled STARZ series Camelot, The Big BadMorgan Pendragon uses her magic to take the form of Guinevere so that her half-brother, King Arthur Pendragon of Camelot, will have sex and concieve an heir with her. She does this after her efforts to usurp him fail and she is deprived of her holdings. Her reasoning behind her act is that "the only way she can be a Queen is by giving birth to a King" - following that logic, Kings are usually the sons of Kings, and the only King in the land is Arthur, which limits Morgan's options.
To his credit, Arthur himself is under the complete impression that he really is having sex with his paramour, and not his half-sister.
In the book, they actually end up getting married, but Ghanima is very firm on the subject: "I will not bear your children, brother," to which Leto replies; "I love you, my sister, but that is not the way my thought tends." Leto, actually, at least claims to be unable to reproduce after forming a bond with sandtrouts. Thus he marries his sister (it helps that he has a committee of ancestor advisors presided over by an Egyptian pharaoh in his head, and that nobles in Dune have a system with both spouses and concubines), but her children (important to Leto's Golden Path) are fathered by a Corinno who also authors the entries of Encyclopedia Exposita. All this said, Stilgar is not amused.
Paul and Alia have incestuous overtones in Dune Messiah. At one point, Alia engages a sparring robot nude, before Paul stops her from killing herself (one wonders how long he was standing there). It's certainly not helped by the Bene Gesserit's clear intention to find a way of bargaining for a way to get Paul and his sister to produce an heir.
The British Soap OperaBrookside had a storyline about a brother and sister who engaged in consensual incest and later split up without any direct or karmic punishment. They later got back together and were written out of the show by establishing that they'd moved to where no one knew them and were living as a married couple.
Weirdly enough, Gotham, with Mad Hatter/Jervis Tetch and his Canon Foreigner sister, Alice. It is one-sided, though, with Alice fleeing from Jervis after mentioning he has thoughts "no brother should have".
Tabitha at one point mentions that her brother Theo is "a monster in the sack", though it's unclear whether they actually had... er, relations, or whether she was saying it for the shock value.
On Lost, Boone and Shannon are stepsiblings who have slept together. Boone was also in love with Shannon.
One episode of House had a young married couple whose similar illnesses were thought to be from exposure to the same environmental factor but turned out to be genetic; they were half-siblings through the husband's (white) father's affair with the wife's (black) mother, and had become attracted to each other as teenagers. The father's determined attempts to keep them apart were misinterpreted as disapproval of interracial romance, and the young couple ran away together before finding out they were related. It is intimated their relationship did not survive the revelation.
In the fourth season finale of Lewis, a young married couple who have suffered three miscarriages discover through testing that they are fraternal twin siblings who were given up for adoption to separate families at birth and who happened to cross paths later in life. As they also discover that they have inherited a rare, incurable, fatal genetic neurological disorder from their father (who committed suicide rather than face a slow, painful death from the condition), they begin a series of murders intended to culminate with the woman they believe is their birth mother. (Except that their birth mother registered under a false name at the hospital where they were born, and she ended up being their first victim rather than the last one.)
A major plot thread in Carnivŕle involves the obsessive, sadomasochism-tinged relationship between preacher-slash-Antichrist Justin Crowe and his sister Iris.
Also, Harper, Alex's best friend who has a crush on Justin, gets mad at Alex at one point because she thinks Alex tries to become Justin's girlfriend.
The movie's ending plays with this. Alex emphasizes that the brother and sister relationship and how much they love each other never change in spite of their arguments. But at this point, Justin has forgotten who she is, which seems to reduce the level of incest. When he tells her "I'd never leave you" it's difficult to not see the scene as romantic.
Somehow played with on Hannah Montana. Miley, in her alter ego as pop star Hannah Montana, is caught going in her own back door by a photographer. To keep her Secret Identity under wraps, she claims to be "visiting a friend"... just as her brother Jackson comes out of the house, leading the paparazzi to assume that they're dating. It gets worse when Jackson realizes that being Hannah's assumed boyfriend has its perks, so he keeps the pretense going even after Miley (who is naturally mortified by the whole thing) repeatedly asks him to stop. He eventually feels guilty and backs down, but not before proclaiming his love for her while jumping on the couch.
Something like this occurs again in an episode where Jackson develops amnesia, and thinks Miley is his girlfriend. She's rightly disgusted.
A series of Saturday Night Live skits during the Eddie Murphy era intimated that this was the case with Donny and Marie Osmond. The impressions weren't really that good, and they were really kind of more disturbing than funny.
And one of them was Elaine Benes. And the other one's career brought even less success.
In the second season of Prison Break, this is the other secret President Reynolds can't allow to become public. The first secret, of course, being faking her brother's death.
Theodore Bagwell "T-Bag's" origin story contains this. More specifically, Bagwell Sr. raped his disabled sister, Theodore was the result, and the rest is history.
In Scrubs, Keith gets mad at Elliott for telling Carla that he made out with his sister in the fifth grade. Elliott says he has nothing to be ashamed about because his sister is gorgeous.
In the HBO series Rome, Octavia, at the behest of her manipulative lesbian lover Servilla, seduces her younger brother Octavian (the future emperor Augustus). There is no evidence for any such affair happening in real life. Made even squickier when it turns out he knew all along that she was only seducing him in order to get something in return, and he was curious as to what it would be. He could have just asked, but...
Subverted in Veronica Mars: Duncan slept with Veronica shortly after (but not, let it be said, because of) finding out she might be his half-sister, which caused him to break off their long-standing relationship without explanation (she wasn't). Veronica didn't find out about her possible parentage - or the fact that they'd slept together (she'd been accidentally dosed with GHB, as had he, which fueled the loss of inhibition) - until much later. They got back together once the whole incest thing was cleared up. (It should be noted that Veronica's taste in men could be charitably described as "quirky".)
To clarify, the order of events is Duncan thinks Veronica is his half-sister, Duncan breaks up with Veronica, Duncan and Veronica have sex while drugged.
Additionally, GHB made Veronica forget the sex, and she woke up thinking she'd been raped (which, incidentally, she had been—but not by Duncan). She tried to report it, but, without evidence or even possible suspects, the sheriff laughed her out of his office with his patented "Go see the wizard and ask for a backbone" line. After a long search, she found out that Duncan had sex with her. Duncan reveals that he remembers the whole thing, but always assumed that Veronica kept quiet because of the embarassment not because she didn't remember.
Averted in Dirty Sexy Money. Nick's father works as a lawyer for the Darling family, has an affair with his client's wife and even fathers a child which is passed off as a Darling. Nick has an affair with one of the Darling daughters in his teens. The show might have ended up fitting this trope - if the showrunner hadn't written INCEST on a board in the writer's room and firmly crossed it out.
Throughout Six Feet Under, bipolar Billy Chenowith is shown to have an unhealthy obsession with his genius sister Brenda, which she shares to a certain degree (they have tattoos of each other's childhood nicknames on their backs, for example). Whenever Billy goes off his medication, things start to get a bit crazy, such as when he tries to carve the tattoo off her back after taking his own off in an effort to "cleanse" himself or when he and Brenda get into an argument and she reveals that their mother showed her "what you wrote about me" several years previously. The audience is never told what it was that she read, but the subtext is pretty obvious. As the show progressed, said subtext rapidly became text, to paraphrase Buffy, when Billy tries to kiss Brenda while she comforts him and in season five, after Brenda's husband Nate dies, she begins imagining seeing him telling her that every man she's ever been with has only been a substitute for Billy, and has a dream in which she and Billy begin to have sex.
Arrested Development turns out to have this in the finale, when pretty much immediately on learning that they've Not Blood Siblings, Lindsay tries to get Michael to marry her. Michael is naturally freaked out. Immediately after finding out about this, Gob hits on Lindsay because he always wants to steal girls from Michael.
In the episode "Family Ties", Michael hires Nellie, a woman whom he believes to be his long-lost sister but who is actually a prostitute. When Gob is told that she might be his sister, his first reaction is that he should have gotten a family discount.
Made even funnier by the fact that Jason Bateman (Michael) and Justine Bateman (Nellie) are brother and sister in Real Life. This is given a nod when Michael blurts out a request to marry him after learning how much money Nellie makes, then says "That's weird on so many levels."
Firefly. In an amusing scene deleted from "Our Mrs. Reynolds", Simon is thrown for a loop by his sister's latest bit of loopiness.
River:(to Shepherd Book) We want you to marry us. Simon: What? We... no! (pause) What? River: Two by two... Everyone has a match, a mate, a dopple. I love you. Simon: No, River... mei-mei... of course, I love you too, but we can't be married. (to Book) She's really crazy! (River kicks Simon in the shin) Ow! Ah, no, I — I don't mean crazy... that's just — you know that's not something brothers and sisters do. I mean, on some planets but only pretty bad ones.
For added squickiness, River then takes a pillow and puts in under her dress and claims "Now we have to be married. I'm in the family way." Puts a whole new spin on that term, doesn't it.
In the fandom, the shipping of Simon and River is called Tamcest, or occasionally, Crazy Space Incest.
When Charlie from Two and a Half Men finds out the girl he's currently bedding might be his half-sister, he immediately stops, saying, "Turns out I draw the line at incest." Since the episode takes place during the holidays, supporting characters call it "a Christmas miracle."
Cold Case: A rising young senator's aide/girlfriend is killed when she learns that her boyfriend's childhood wasn't just "rough", it was "make your sister look and act like your wife-rough". To the senator's credit, it was an accident and the coverup was his sister's idea.
The children of two unequally wealthy families mistake their parents' disapproval of their relationship as having to do with their respective social status. In fact, they're half-siblings through the father's infidelity. The SVU is called in when the girl, who was pregnant, is murdered. When the boy learns of it later, he immediately throws up, crying, "I had sex with my sister?!" Stabler assures him that it's not his fault (since he had no idea) and encourages him to keep it together for the sake of the girl's younger brother, who is his half-brother, as both the mothers involved were going to jail for separate incidents in the case and their father is dead. Poor guy.
There is another episode with a Con Couple turn out to be brother and sister. Worse of all they are even twins who justify their relationship by stating "they share the same soul".
In still another SVU episode ("Identity"), a pair of adolescent twins are forced to enact incestuous sex by their therapist, who claims it will help encourage the girl twin to think, respond and behave like a female, because she was born a male and had a sex reassignment in infancy after a surgical accident. This, of course, really happened to the late David (Brenda) Reimer and his twin brother. The twins are emotionally close, but not that close.
A CSI: Miami episode played like the Law and Order episode above mixed with Marmalade Boy: a man has two families and his son and daughter fall in love. The Squick comes in after they learn they're half-siblings and stay in the relationship anyway. The half-siblings/couple kill dad when he wanted their unborn baby aborted.
In Angel, Gunn once was accused of letting his sister be turned into a vampire due to incestuous feelings for her. (It Makes Sense in Context.) Seeing the scene where she tries to turn him, and most of that episode, one understands where people got the idea there was something more there.
In the Friends episode "The One with the Inappropriate Sister," it's heavily hinted that a man that Rachel is dating has a "special relationship" with his sister. They even take baths together. Rachel eventually breaks up with the guy because of this.
In "The One Where The Stripper Cries" it is revealed that Ross accidentally kissed Monica, thinking she was Rachel, while he was in college.
An episode of Private Practice had a married couple who found out they were half-brother and sister (same sperm donor as a father), and then wanted to stay together and have the sister's tubes tied. The writers then pulled a Debate and Switch when it turned out the brother knew before the couple got married.
The founding and series naming story arc of 1980s Australian prime-time soap Sons And Daughters had two non-identical twins, separated at birth and moved to different cities, fall in love. It's only revealed that they are siblings after the engagement. Squicked a generation of Australian soapie viewers as it had been implied their relationship was not platonic by any means.
On Passions, sisters Whitney and Simone were both in love with Chad, who viewers were led to suspect was their half-brother, as the girls' mother, Eve, had a child with Julian Crane, but the child was kidnapped. Eventually, Chad and Whitney started a relationship (and Simone became a lesbian) and Whitney got pregnant. Immediately thereafter, she and Chad learned that they were, in fact, brother and sister because Chad was Eve and Julian's missing child. After about two years of angst, they found out that Chad was actually the child of Julian's father and Eve's adopted sister, meaning that Chad and Whitney were Not Blood Siblings after all. As if to show that the trope had been averted only because the writers thought they could push the envelope further, they then brought on Eve and Julian's actual child, a psychopathic hermaphrodite named Vincent (who had been living in Harmony as the established character Valerie for several years), who proceeded to have an affair with his uncle Chad (both as Valerie and as Vincent, at separate times), rape his half-sister Fancy, and seduce and become pregnant by his own father.
In the BBC series of Robin Hood...it's complicated. Technically no actual incest actually occurs, but with the introduction of twolong lost siblings, things get rather awkward. Robin starts a relationship with Isabella, and although the two of them aren't related by blood, it's later revealed they share a half-brother, and that Robin's father planned to marry Isabella's mother, meaning they were almost step-siblings. Furthermore, when the time comes for this long lost brother to introduce himself to his half-sister, the writers instantly make the relationship explicit in the hopes of sinking any incestual vibes. It doesn't work. Also, at one stage (during a heat wave) Isabella bathes Prince John's head with a cold pack, cooing the words "hot, hot, hot." whilst smirking at Guy, her full brother, over John's head. In a later episode, Isabella drugs and gags Guy before tying him to her bed, and even later, Guy strokes her hair in a surprisingly intimate gesture.
In "The Killings at Badger's Drift", an episode of the British television series Midsomer Murders, an old woman discovers a brother and sister in an incestuous relationship and they murder her to keep her quiet. Later, a busybody and her son, who likes to know everyone's secrets and is not averse to a spot of blackmail, end up dying as well. In the end, the brother and sister end up committing a double suicide.
In another, a couple of Hippie Parents throw out their eldest children in The '70s, as their open-mindedness didn't extend to incest. They crash the car, but the sister survives, and settles down not too far away. Years later, the remaining sister is secretly banging the man she doesn't know is her nephew.
One of the main plots on Kinderen Geen Bezwaar ("No Objections from the Kids"). It's of the step-sibling variety, which isn't too clear if you didn't see the first few episodes...
Harper's Island: Abby's mother is also Henry's mother. His father is her ex-boyfriend John Wakefield, who later becomes Ax-Crazy and kills her after a 17 year prison stretch for something he didn't do. Henry and Abby were introduced as children but never told they were half-siblings. Henry fell for Abby, later found out his true parentage and decided the only way to be together would be to live alone on the titular island and be believed dead. He kinda ruins things by murdering 30 people in the process. Of course, Abby isn't happy with the arrangement and stabs him to death with his own knife. Ouch.
On Casualty, nurse Anna falls for someone only to discover that he's actually her long lost half-brother (given up for adoption). She tries to break up with him, but they eventually end up back together, leading to a pregnancy which they suspect is a very bad idea... Then she dies horribly, neatly getting rid of both the abortion and incest moral dilemmas.
In an episode of The Bill, a sergeant and a constable (who is struggling with his own sexual repression) happen upon an apparent couple while searching for a kidnapped child. Turns out that the nervous couple have nothing to do with the missing girl, but are skittish for an entirely different reason. The constable is disgusted and wants to bring the two in, but the sergeant points out that they should be left in peace, as the siblings are miserable enough as they are- hiding from the world and taking care of their senile mother, who berates them and their relationship every chance she gets - and they're not doing any harm to anyone else.
Very strongly hinted by Tubbs and Edward in The League of Gentlemen, to the point where Edward mentioned "their mother".
Steph Stokes was sexually abused as a child by her brother in Emmerdale.
The soap opera Hollyoaks featured a storyline where Rhys met and started dating new-in-town girl Beth, only to find out after the death of his 'uncle' Noel (who was actually his father) that they were half-siblings. They stopped the relationship for a while, and Beth even became engaged to Rhys' best friend, Gilly. However, during this time it is strongly implied that the two still have feelings for one another and they eventually restarted their relationship in secret. After the relationship was discovered, Rhys and Beth ran away, but they were in a car accident and Beth was killed, neatly dealing with that storyline.
Declan and Fiona on Degrassi: The Next Generation are twins with an oddly high level of sexual tension, to the point where in a jealous rage Fiona makes out with him in front of his girlfriend.
The idea that Declan and Fiona were intimate was a joke thrown around the Degrassi fandom for most of their introductory season. It wasn't until The Movie that people realized it was (sort of) canon on Fiona's part (lines like "Boyfriends are temporary; brothers are forever" hammered the point home).
Clare and Jake have a weird example of this. When they start seeing each other, they're not related in any way. Then their respective parents start dating. Then they get married... They ultimately break up for good, but not until after the two families are merged.
Narrowly avoided on All My Children when Erica Kane was engaged to Mark. Her mother stepped in Just in Time to prevent them from going any farther, and dropped the bomb that they were half-siblings through one of Erica's father's many liaisons.
This also happened with Erica's eldest daughter, Kendall Hart-Slater, not once, but twice! First with her half-brother Trey Kenyon whom she felt an instant connection and attraction to but didn't know why, and who came to town specifically looking for his family, fell in lust with her and almost slept with her before it was revealed that she was his sister. The second time was with her other half-brother Joshua Madden, who looked exactly like her but male, and who spent the majority of his time shamelessly flirting with her. His adopted father knew he was her brother, was keeping it a secret because Josh's adoption was far from legal as he was literally stolen from Erica's womb and implanted by Dr. Madden into his barren wife, and became increasingly concerned and unhinged as he realized that Kendall and Josh were becoming closer and closer and heading from friend territory into sexual partner land.
One of Masterpiece Mystery's excellent dramas based on the stories of Miss Marple had her helping a young woman who was starting to believe she was either being haunted or being tormented by memories of a previous life. Her mother died when she was very young and her family history was convoluted at best, but she was trying to move ahead and had even reconnected with her long-lost uncle, her mother's brother. Turns out that the young woman's mother had moved to India to get away from her brother because he had an unhealthy obsession with her. When she returned to England under an assumed name to be with her husband and daughter, he found her, and when she spurned him again he murdered her.
The Finale of Smallville averts any chance of it having happened between Tess Mercer and her half-brother Lex Luthor. Given her line a few seasons back, "I loved you, you son of a bitch," one wondered just what sort of relationship they had. However, Lex reveals in the finale that he knew all along that Lionel was her father.
Kingdom Hospital. Hinted with Abel and Christa. ("We're all brothers." "We're all sisters.")
An early episode of Heartbeat saw the police investigating a schoolgirl who had had an illegal abortion. They assumed the father was a boy from her class, even though it was obvious to the viewers that he was just a harmless loser with a crush on her. In the last five minutes, as she left town, the girl told the lead policeman's doctor wife in confidence that the actual father was her brother. Most viewers probably guessed this, since he was the only other person in the episode. (If the episode wasn't disturbing enough already, the schoolgirl was played by series star Nick Berry's real-life wife. The conversations at home during filming must have been interesting to say the least.)
Pretty Little Liars has this between stepsiblings Toby and Jenna Cavanaugh. Oddly, Jenna was the one pressuring Toby into sleeping with her, and he wanted it to stop. Unfortunately, nobody is willing to listen to his side of the story, and everyone assumes that he is the abusive one.
In It's Always Sunny in Philadelphia, the McPoyles are an entire family that participates in incest. The brothers "bang each other", and both are seen making out with their sister passionately in public.
When the gang become just like the McPoyles in one episode, Dennis and Dee begin leering at each other lustily.
In the episode "Who got Dee pregnant?", Dennis is squicked by the possibility that he might have accidentally had sex with and impregnated Dee through a convoluted series of events at a Halloween party. His concerns turn out to be unfounded; however, it's never outright stated that they didn't have sex, only that he's not the baby daddy.
Frank also fools around with his (non-blood) niece in an attempt to sleep with his dead brother's wife.
Although this could have been Getting Crap Past the Radar. Word of God said they backed off rather suddenly on this ship to start the Arthur/Gwen ship, but in hindsight, touching on a relationship with Arthur and Morgana way before they confirmed the two shared a father was probably the only way to show their legendary incestuous relationship on a family show.
In an episode of Criminal Minds, a town's mysterious arsonist turns out to have been beaten and driven out by the townsfolk years prior because a rumor was spread that he was romantically involved with his sister. The twist ending, though, is that the rumors are completely true, at least to a point; it's never explicitly stated they had a physically intimate relationship. They just danced way too close, looked way too long, and had romantic feelings for one another. The sister, however, moved on with her life after her brother was sent away; her new husband was among his arson victims, and when her brother reveals himself and his feelings for her, she's horrified.
In a more recent episode, a brother and sister pair are poisoning women they think to be the Devil's wives. The brother is extremely protective of her, not letting her go out or live a normal life. When the brother ends up thinking his little sister is one of the Devil's wives as well, he poisons her... but not before giving her a kiss on her lips. Prior to that, he'd also danced with her to a romantic song in a prom ball he threw exclusively for her, since he wouldn't allow her to go out to her actual prom.
In an even more recent episode, a psychotic killer was violently murdering members of two old rival families in the town. It turns out the mother of the first victim from one family and the father of the first victim from the other family are brother and sister; the sister just left town and changed her name before marrying into the rival family. The reason she left town was because she got pregnant at 16... with her brother's child. That child, of course, became the serial killer that had been terrorizing the town.
In The Borgias, Cesare and Lucrezia Borgia are prime examples of this trope. It's fairly obvious that they do not have a typical sibling relationship. He's threatened to murder her husband, compared themselves to the star-crossed lovers Abelard and Heloise, stated that he would die if anything happened to her, and when asked by Lucrezia if he loves God, replied, "More than I love you?" She's stated that she could never love a husband as much as she loves him, once said that she, um, missed his hands, spies on him having sex, and, like him, cannot keep to her personal space. They often spend time bumping noses and cuddling and dancing while discussing terms of romantic endearment. The relationship is becoming VERY physical in season 3, when they have sex in the third episode of the season.
Of course, this is almost a Foregone Conclusion, because in real life, it's hard to know whether they had this kind of relationship — let's just say that the Borgias' enemies claim they had.
Borgia — an unrelated show about the same piece of history — also addresses this. Handled in a very interesting way, it's ultimately subverted because Cesare and Lucrezia never actually have sex, which is the definition of incest. However, they get very close to having sex multiple times, and they have a romantic dynamic. And while they never have sex, it's more due to circumstances than lack of interest. The more-likely historical truth (that people made up rumors they were incestuous to hurt the Borgias) is also present in the story, and discussed. These rumors appear to be made up separately from the truth.
And one time, Juan briefly tries to rape Lucrezia.
Almost happened with a story arc in Ugly Betty. When Amanda finds out that her biological mother was Feye Sommers, who had a long standing affair with Daniel's father, both she and Daniel are squicked out by the fact that they could be half-siblings because they had previously had a sexual relationship.
On the Dexter TV series, Debra kisses her brother Dexter, although it turns out to be a dream. Partially justified by the fact that he's an adopted brother. Then it turns out Deb loves him romantically.
The Trinity Killer's Start of Darkness was his sister slipping and bleeding to death in the shower after she spotted him watching her. He would later insist it was actually innocent, but...
Incest was also a major theme in the sixth season , with the season's Big Bad having a very strange relationship with his sister.
A major plot point: the fraternal twins Jaime and Cersei Lannister have been in a relationship since their childhood, and all of Cersei's children were fathered by Jaime, not her husband the king Robert Baratheon; she also sleeps with their cousin Lancel when Jaime is away fighting the Starks. Finally, the parents of said twins and their brother Tyrion were themselves first cousins. It's a wonder that the only genetic problem in that generation was Tyrion's dwarfism. Bran Stark's discovery of this dark secret is one of the catalysts of the series.
Viserys was uncomfortably possessive of Dany's body, going so far as to strip her and fondle her, while commenting on her figure. Cersei mentions later that House Targaryen, to which they both belong, has practiced incest (mainly of the brother-sister variety) for three hundred years "to keep bloodlines pure", similar to the royal family of ancient Egypt. However, Aerys and Rhaella represent the first time it had been practiced in more than a century. Even before that, it was by no means a given; Aenys I married a Velaryon, Maegor I many women but the closest relative was his niece (and none of them had any children, anyways), Viserys I an Arryn and a Hightower in succession, Rhaenyra I her cousin once removed (paternal grandfather's brother's daughter's son) then her paternal uncle, Aegon III his cousin followed by a Velaryon, Viserys II a Lyseni from Essos, Maekar I a Dayne, and Aegon V a Blackwood. Aerys II's father Jaehaerys II did marry his sister, but he has been adapted out of the TV series. That means that the only monarchs to marry their sisters were Aegon I (Dany's greatx10 grandfather), Jaehaerys I (her greatx8 grandfather), Aegon II (no direct relation as all of their children died but technically her greatx6 uncle), Baelor I (unconsummated, and therefore no direct relation, but her cousin five times removed), and Aerys II (her father). That said, many splinter branches of the family did marry their siblings, but these are the only monarchs to do so and two of them had their branches die out. In the novels it is implied that Viserys and Daenerys would have been married when they reached the appropriate age. In interviews, the actor who portrays him describes Daenerys as his "sister, wife, and daughter" all in one, often mentions that his character feels as though he "owns" Daenerys due to the fact that he would have married her if he still had the throne, and is even somewhat jealous of having to give her up to another man.
A bit of unintentional (on one side) occurs when Theon Greyjoy returns home to the Iron islands and meets a young woman on the docks who says she'll take him to his ancestral castle of Pyke. On the way there, he flirts endlessly with her and starts fingering her on the horseback ride. Once he got to the castle, however, Theon gets a rude slap in the face when he learns that the young lass is his sister, Yara Greyjoy, and she was trying to see what kind of man he was, and is less than impressed by what she saw.
In an episode of Mongrels, Nelson falls for his sister while she's in heat, going as far as to marry her, and spend the wedding night as you'd expect him to. Of course, once his sister comes out of heat, Nelson immediately regrets his decision.
Wolf of Outrageous Fortune fathered a child with another woman. His family discover this after he goes on the run when his will is found. Unfortunately, his daughter Pascalle has already slept with the man in question.
Nearly happens in Young Dracula when Ingrid gets very close to kissing Malik until he turns her down and then reveals in the next episode he's her half-brother.
In Sons of Anarchy Jax nearly has sex with his Irish half-sister before their relationship is revealed to both of them. Then again in Season 5 Nero's bordello manager Carla turns out to be both his half-sister and to have a Yandere level crush on him. Whether they ever had sex in the past is left ambiguous.
In Hercules: The Legendary Journeys, Young Hercules, and Xena: Warrior Princess, they actually acknowledged that several of the gods were siblings and having sexual relations with each other, just like in the myths. Amusingly, in the Xena episode "The God You Know", Aphrodite goes crazy and hits on several people including Ares. Ares freaks out and tries to snap her out of it, saying he's her brother and that she's not herself, when Aphrodite and Ares regularly have affairs in the myths.
In an aversion, the second season of Xena: Warrior Princess went out of its way to establish that Xena was not Ares's daughter (though that was the simplest way to explain her general badassery) after hinting that she might be during the first season, perhaps because someone realized that Xena and Hercules had been lovers on the latter's show, and since Hercules and Ares were half-brothers, they didn't want to raise the spectre of incest (or cut off possibilities between Xena and Ares)
New Tricks: A case occurs in "Couldn't Organise One", although here it turns out to provide an alibi for the murder.
Factored into one storyline on the soap opera One Life to Live, when a character named David showed up in Llanview claiming to be the biological son of the late Victor Lord. Victor's daughter Vicki was less than enthused, but her half-sister Tina quite liked David. David quite liked Tina, too... entirely too much for a brother. Making the situation even more awkward, David's claim was that he was Tina's full brother, sharing both mother and father with her. They eventually slept together, and only afterward did he admit he wasn't her brother at all.
This is pretty much the theme of the Masters of Horror episode "Imprint". It's also one of the least disturbing elements in it.
In a major twist it's revealed that the disfigured prostitute's parents were actually siblings. They were run out of their town and lived in poverty for the rest of their lives. The brother turned cruel and abusive towards his sister and was killed by his inbred daughter after he is implied to have raped her.
Christopher had a relationship with his younger sister. It's later implied that he raped and killed her and was driven insane due to it.
In one episode of Red Dwarf, after Lister talks about how he's an orphan who was found in a box under a pool table in a pub, this is Rimmer's suggestion for who his parents are. Everyone starts laughing. Except Lister.
Though given that Lister is his own father, this means he's the product of a mother and her son, a related trope.
I, Claudius depicts the rumoured incestuous relationship between Emperor Gaius Caligula and his three sisters, Drusilla, Julia Livilla, and Agrippinilla. His grandmother, Antonia Minor, finds him nude in bed with Drusilla when they are just children, and after he becomes Emperor, she becomes pregnant by him. The relationship ends gorily when he kills her by cutting their unborn child out of her womb and eating it.
The Caesars, like the later I, Claudius, shows Caligula in an incestuous relationship with his three sisters. Even before the fever which causes his insanity, he lustily asks Drusilla's husband how "those lovely legs of hers" are. In this version, he accidentally strangles Drusilla during sex and banishes Julia Livilla and Agrippinilla for alleged treason.
Another Period has upper-class siblings Beatrice and Frederick Bellacourt regularly jumping into bed with one another. Never mind that Beatrice has a husband (who is openly gay, and having his own affair with her sister's equally-gay husband). Strangely, when Sigmund Freud (yes, that Sigmund Freud!) catches the two in bed together, he doesn't think it's a big deal.
The 2015 mini-series Flesh and Bone has this as a central theme. Though in this case it's made pretty clear that it was rape - and a large part of the protagonist's Dark and Troubled Past.
On Shadowhunters, Jace and Clary, who have romantic feelings for each other and have kissed numerous times, are actually brother and sister as revealed in the episode "Blood Calls To Bloods".
On Homicide: Life on the Street, the detectives discover that their murder victim was having an affair when his widow shows them pictures that a private investigator sent to her. She tells him that the other woman is her husband's half-sister.
The Young and the Restless. Victor can't put his finger on why he disapproves of his daughter's new boyfriend, until he realizes that the man is the son of a woman who was his mistress years ago. He does the math and realizes that there is a possibility that he could be the man's father. Unfortunately, the two have already eloped—and consummated the marriage. Fortunately a DNA test shows that he isn't the man's father after all.
One Life to Live. David Vickers comes to town with evidence indicating that he's the half-brother of local heroine Viki Lord and the full brother of her half-sister Tina. While Viki has her doubts and reservations, Tina instantly likes him—or is perhaps instantly attracted to him, an attraction that is reciprocated until they finally end up sleeping together. Tina is horrified at having committed incest and even more disturbed that David isn't—until David admits that he actually is not her brother. Tina's relief at not having committed incest is quickly eclipsed by realizing that she still slept with a lying con-artist.
In korean drama "Moon Lovers" princess Yeon-Hwa is dating and later engaged to one of her brothers, while being romantically interested in the other, and having a rather suggestive, lady-Macbeth-type relationship with the third one. Later she marries the brother she desires and becomes his queen.
Taboo: James and Zilpha once had a sexual relationship, and they still seem to be in love with each other at present. He even starts visiting her in her dreams (maybe) to have sex. It's later consummated for real after she kills her abusive husband.
Hemlock Grove: Roman somehow ends up sleeping with two different half-sisters. The first time, he was brainwashed and had no recollection of the incident. This resulted in pregnancy, but she later died in childbirth. The second time, the other sister—who was a vampire just like him—knew that they were siblings but chose not to tell him until after the fact.
Series/Juliet Bravo: A drunk who fell asleep in a barn belonging to an isolated farm tells the police that the couple who own the farm are keeping a girl chained up: the police investigate, and the couple tell them that the girl, their daughter, is "simple" and confined for her own protection; on examination the girl turns out to be, not learning-impaired, but profoundly deaf; the couple admit to being brother and sister, and say that their daughter's handicap is divine punishment.