Blades of Glory: villains Stranz and Fairchild Van Waldenberg fairly smolder with sexual tension for one another until they finally fling themselves at each other just as the police are hauling them away. It's OK, though - the actors are married in Real Life.
El Cid: Prince (later King) Alfonso seems to have a bit of this going on with his sister Princess Urraca. Historians have speculated that the real Alfonso and Urraca may have been incestuously involved with each other.
In the Romanian movie called Love Sick (Legaturi Bolnavicioase), there is a college girl who sleeps with her brother and is in love with her best friend. Who is also a girl.
The Devil's Rejects features Otis and Baby, who refer to each other as brother and sister (even though they aren't blood-related) and are quite comfortable acting sexually around one another. Otis demands to know why one of their victims is not turned on by his "sister" at one point. The actor who plays Otis has admitted to reminding himself multiple times on set that she was the boss's wife (smoking hot but unavailable), and her Real Life brother has a cameo in the movie (as a cop).
Euro Trip. Twins Jenny and Jamie chug down absinthe at a nightclub, become immensely pissed, and start making out with the first person in their reach...realizing far, far too late that they're actually making out with each other. Hilarity Ensues.
Worst. Twins. Ever.
Innocent Lies, a British thriller, has this between Celia and Jeremy. They flirt rather openly (including eating melted chocolate off each other) despite Celia being engaged. We later hear her tell a detective all the sordid details of her Squicky sex life with her brother. The two even resort to murdering their own mother to keep their relationship a semi-secret.
In Joe Dirt, Joe realizes that a woman who is intensely attracted to him may be his long-lost sister, but because he's normally such a loser romantically, he doesn't tell her until after they have sex. Fortunately, it turns out her brother wasn't long-lost, they found him again almost immediately, meaning she's not related to Joe. This means they can have sex again, but Joe can't get it up anymore - until they role play being brother and sister. In the present day setting of the movie, Joe then admits he made that last part up to gross out his listeners.
Or did he?
Ginger Snaps heavily links lycanthropy with sex; this becomes particularly Squicky when Ginger suggests Bridgette should let Ginger bite her so they can be their own pack.
The sequel Ginger Snaps Back The Beginning, which places the characters in the eighteenth century... somehow... makes this even more explicit, with Bridgette ultimately choosing to be with Ginger over any other considerations. It's also implied in the film that they had killed their parents to be together...
Gladiator: Emperor Commodus plans to do this with his sister whether she wants to or not (she doesn't), but never gets the chance. That and threatening her son make a good Kick the Dog moment, though.
Hellboy II: Prince Nuada seems to have a lot of sexual tension going with his sister Princess Nuala. The presence of implied incest was confirmed by Nuada's actor.
The House of Yes: the twins blatantly admit to their incestuous behavior, much to their younger siblings' disgust.
My Chauffeur: In this otherwise forgettable film, the male and female lead characters hump like bunnies. The next day they find out they might be brother and sister. Hilarity Ensues.
In National Lampoon's Class Reunion, what turned antagonist Walter Baylor into a crazy az murderer was a prank where he was set up to have a makeout session with what turned out to be his own twin sister.
Oldboy: In this Korean thriller, it turns out that the reason the protagonist is imprisoned for ten years in a hidden private jail is that while in school, he told people about seeing the antagonist having sex with a girl who (unbeknownst to the protagonist) was the antagonist's sister. In a further twist, it's revealed that the woman that the protagonist falls in love with after getting out of prison is in fact his own daughter, and that the antagonist arranged his captivity specifically to produce this outcome.
The Other Boleyn Girl: Anne Boleyn miscarries her second child, then asks her brother George to try to impregnate her, though in the end he's incapable of doing the deed.
Repo! The Genetic Opera has Luigi and Amber Sweet. While all the Largo siblings hate each other, these two have some serious UST going on. Pavi even comments on it in "Mark It Up".
Pavi: My brother and sister should— Luigi: Pavi! Shut the FUCK up!
The Rocky Horror Picture Show: Riff-Raff and Magenta certainly seem to have a closer relationship than most siblings (just look at a certain phrase above...). The same actors, playing different characters in the execrable sequel, Shock Treatment, have a much more explicitly BSI relationship.
Say It Isn't So Uses this as a joke for almost the whole film. Jo(Heather Graham) and Gilbert(Chris Klein) fall in love. Jo's parents aren't too happy that she isn't dating a rich guy, so they come up with a plan to convince her and Gilbert that they're brother and sister, so that they can become rich. Gilbert realizes they're not related when Jo's REAL brother shows up, then Hilarity Ensues when Gilbert repeatedly goes to great lengths to convince Jo that they aren't related.
After they get married, a brick falls out of the sky, in the shape of Susanne Sommers, who turns out is Gilbert's real mother. Gilbert is so happy, until Jo reminds him of what he told her earlier... using posters of said celebrity as masturbation fodder in his teen years.
Scarface: Tony Montana's violent protectiveness toward his sister Gina has definite elements of this, inspired by the 1932 film it was a remake of, which had its title character carrying on the same kind of relationship with his own sister. This, combined with the effects of his increasing cocaine addiction, culminates in Tony gunning down his best friend Manny after catching him with her (and after she and Manny had gotten married, no less). Gina confronts Tony near the end about it as his mansion is being besieged by Sosa's killers and tries to kill him, but Sosa's men get her before she can reach him. This, among other things, is what finally drives Tony to take up his M-16 and go out with guns blazing.
Star Wars: Many a fan has commented that if indeed Leia had "always known" that Luke Skywalker was her brother, that casts the full blown snogging in Empire Strikes Back in a whole new light.
The novel Splinter of the Mind's Eye was the most blatant example. Han Solo didn't even appear in this book (which supposedly took place, chonologically, before Empire Strikes Back and the two seemed very much a romantic couple. (Of course, the author had no idea at the time that the two characters were siblings; Lucas made that decision later.)
Also, a Robot Chicken segment where you see the two lying in bed with one another, before Leia comments "That was so wrong".
A Very Brady Sequel: Averted (barely) when Greg and Marcia bravely fight an attraction inspired by the real-life sexual tension between Maureen McCormick and Barry Williams in the original show.
Parodied in Two Stupid Dogs in an episode where the dogs go to live with a family much like the Bradys. The teenage brother and sister get into a fight, before the dad tells them to kiss and make up. They proceed to do so literally by making out.
Of course, Greg and Marcia aren't blood relations. Theirs is a blended family, formed when Mike, father of three boys, married Carol, mother of three girls.
In The Seeker, there is a distinctly unsisterly vibe towards Will from the youngest sister.
In Amityville II: The Possession Sonny Montelli (based on Ronald Defeo, Jr.) is influenced by the evil in the house to seduce and have sex with his sister.
In the adaption of Flowers in the Attic, they removed much of this trope when it came to Christopher and Cathy, but not completely. They are obviously very very close, are awfully comfortable getting naked around each other, and at one point he washes her back for her while she was in the bath.
The Clive Owen movie Close My Eyes is all about this: he has an incestuous sexual relationship with a sister he's not seen for years. It seems to be some kind of metaphor for the decadence and corruption of Thatcher's England.
John Sayles' Lone Star: after they've begun a new relationship, the main character and his love interest find out the reason why his much hated father kept them apart while they were teen was because they were half-brother and sister. Since she is sterile, they decide to move somewhere their biological connection won't be known and go on with the relationship.
In the 1988 remake of D.O.A it is revealed that the murder victim and his long-term girlfriend were brother and sister. Neither of them knew it.
The Lion King may or may not contain a half-sibling example with Simba and Nala. While any shared parentage is never made explicit, the movie never addresses the fact that Mufasa, as king of the pride, would normally be the one to father any cubs, and that Mufasa and Scar (who is stated never to have fathered any cubs) are the only male lions anywhere to be seen.
This is a plot point in the film Angels and Insects.
Very explicit in the 2006 Poe adaptation of The Fall of the House of Usher. In this case, the incest was a family tradition that already ran over 5 or 6 generations.
In the 1995 film Haunted 1988 (not in the original book, though), Christina has an unusually intimate relationship with her brothers, particularly the older one who uses her as his nude model. As it turns out, the adult siblings were having sex regularly, their mother found out and killed them all, and everyone in the house is a ghost reliving their incestuous past.
In Tromeo And Juliet, the titular lovers find out they're really brother and sister, but decide "Fuck it", and continue their relationship anyway.
At the beginning of the movie, Sammy, one of Juliet's cousins, hits on his sister Georgie and the genetic ramifications of this trope is discussed. Which, as Oancitizen pointed out, foreshadowed the ending.
One of the main plot elements of The Dreamers.
In Enter The Void, incest between Oscar and his sister is heavily implied.
Richie and Margot's intense desire for one another initiates several important plot elements of The Royal Tenenbaums. Of course, as Royal would always point out, she's adopted.
This World, Then the Fireworks (1997) a film noir with Billy Zane and Gina Gershon as twins.
The twist in the Australian film Beautiful Kate is that Kate had sex with her twin brother Ned and older brother Cliff. Cliff then killed her in a car crash and committed suicide out of shame.
In the 1982 Cat People, Malcom McDowell insists that his full sister Natassja Kinski have sex with him as the only way to continue their family. (They will turn into a panther if they mate with a normal human and must kill before they can be restored.) He says that their parents were full siblings as well.
In the 1979 Caligula, the title emperor has an affair with his sister Julia Drusilla, and plans to make her his empress.
Backstory for the protagonist's love interest in The Woodsman.
In the 2009 Mexican drama Daniel & Ana, pornographers kidnap two siblings and force them at gunpoint to have sex on camera. Following their release, Ana, now dreading her upcoming marriage, enters therapy, while Daniel becomes consumed with lust for his sister.
After all the goat screwing, mass murders, and rapes; it turns out the serial killer couple in Island Of Death are brother and sister.
In the deeply fucked-up Dogtooth (Greek title: Kynodontas), this is taken to an extreme: the parents let the son choose which sister he wants to have sex with after the woman Dad had been bringing home for him breaks the psychotic rules of the household and is ousted. She doesn't enjoy it.It Gets Worse.
A possibly unintentional example, but in Transformers, according to Sam's family tree◊, the Witwicky family tree consists of two generations of inbreeding. This however might just be the fact that Sam did a terrible job researching.
Used very disturbingly in Henry: Portrait of a Serial Killer. This trope is something of a Berserk Button for Henry, so when he walks in on his crime partner raping his own sister, he kills them both.
Harry+Max is a film centered around two brothers who start having a sexual affair while dealing with being famous singers being exploited by their family and agents.
In I Start Counting, based on a novel by Audrey Erskine Lindop, a girl of 14 is in love with her 32-year-old adopted brother, even when she suspects he's the local sex killer. She makes an advance to him but he refuses.
This is brought up, but dismissed, in On a Clear Day You Can See Forever. Todd Pringle points out that, since he is just as much in love with Daisy as Doctor Chabot is, perhaps he (Todd) might make a suitable match for her. When Dr. Chabot points out that Todd is Daisy's brother, Todd responds with, "Only by marriage... she's my step-sister, not my blood-sister."
In The People Under the Stairs, "Mommy and Daddy" (the villains) are eventually revealed to be incestuous brother and sister - and that it's a family tradition going back who knows how many generations.
In Sister My Sister, the protagonists are sisters who work as live-in maids for the same employer and eventually develop a sexual relationship. Based on a true story, too.
Blood, a 2004 Canadian movie is all about the dirty talk between sister and her brother, just released from jail. The whole action (nothing explicit) in the movie takes place in a room. But the movie has incestuous overtones.
From Beginning To End is a Brazilian film about two half brothers who begin an incestuous affair. The older brother asks the younger to marry him shortly before the younger one leaves for three years to train in Russia for the Olympics.
A big part of Miguel's mental unstability in Bloody Moon is his lust for his sister Manuela.
In The Crow, this is the nature of Top Dollar's relationship with his half-sister Myca.
Izzy from Sick Girl initially seems to just idolize her older brother, but as the film progresses it's revealed that she is in love with him, to the point of fantasizing that they are a married couple, and their younger brother is their son.
The Freudian Excuse of the villain from Scrapbook is that he was raped by his sister and brother.
The deformed cannibal siblings in Wrong Turn 2: Dead End. The girl gets jealous and violent when she discovers her brother masturbating to the sight of a sunbathing woman.
Although whether or not they actually had engaged in incestuous relations in the past is purposefully kept vague (according to the actors and producers), the mutual sexual attraction between criminal siblings played by Eric Bana and Olivia Wilde is most definitely intentional in the movie Deadfall and in fact drives a lot of the plot. +
In Thats My Boy, Donny discovers that his son Han's fiancee is having an affair with her brother. He tells Donny who calls off the wedding.