Somone noted the following relationship in Little Boy Blue
Ryan Phillippe has sex with Nastassja Kinski in a forced mother/son relationship imposed by abusive father. He finds out that he fathered his two little bnothers.
But Phillippe eventually discovers that Kinski is not his real mother, so his sons are not also his brothers. His abusive "father" (Ray Savage) is actually a stepfather, who was castrated in Vietnam. Years earlier he had hitched a ride with Phillipe's real mother, murdered her, and stolen the infant Phillipe and raised him as his own son. When Phillipe reaches puberty, Savage forces him to have intercourse with his "mother" Kinski while Savage watches. Kinski, who obviously knows that Philippe is not her son, goes along with Savage partly out of fear and partly because she likes having sex with Phillipe. The big reveal occurs when Phillipe learns that his real mother was murdered by Savage.
Other movies with actual incest between close blood relations that I don't recall being mentioned include the following:The Weight of Water
(2000) -- two brothers and their wives/girlfriends (Sean Penn, Elizabeth Hurley, Catherine McCormick, Josh Lucas) take a sailing trip to an island off New England where McCormick tries to solve the mystery of a murder that occurred there 100 years earlier. It turns out that a brief incestuous relationship between a brother and sister helped cause the murder (in a series of flashbacks, Sarah Polley and Anders W. Berthelsen play the adult siblings, and Rita Kvist and Jan Tore Kristoffersen play the couple as young adolescents when the illicit affair occurred).
Someone earlier mentioned an incestuous moment in I Claudius
between Caligula (John Hurt) and his grandmother Livia (Sian Phillips), but the more explicit and historically accurate onscreen affair of course occurs between Caligula and his sister Drusilla (Beth Morris).
In the Australian fim, Beautiful Kate
(2009), the title character has sex with both her brothers, which leads to her death, the death of one of the brothers, and to the estrangement of the other brother from the rest of his family.
In Angel Heart
(1987) a private investigator (Mickey Rourke) has sex with a young mulatto voodoo priestess (Lisa Bonet) who turns out to be his daughter. He also happens to kill her without being aware of it until the big reveal at the end.
In Innocent Lies
(1995) a brother and sister (Gabrielle Anwar and Stephen Dorff), who killed another sister when they were children, later kill their mother and eventually consummate their obsession with each other by having intercourse, until the brother tries to kill that sister during the act.
In Louis Malle's Murmur of the Heart
an adolescent boy with a heart condition has a one-night stand with his young and sexy mother while they are vacationing together at a resort hotel.
In The Quiet
(2005) a father (Martin Donovan) has been molesting his teenaged daughter (Elisha Cuthbert) for years until she eventually kills him with the help of her neurotic friend (Camilla Belle) but lets her drug-addled mother (Edie Falco) take the blame for it.
Someone may have mentioned Alpine Fire
(1985 aka Höhenfeuer) which depicts a family of four isolated on a cold, bleak Alpine farm where the older sister has an affair with her deaf-mute brother which leads to her pregnancy and a tragic confrontation with the father.
(2009) -- one of the weirdest films ever made -- a brother and sister have sex, and then she tells him she'll kill him if he ever touches her again.
in The War Zone
a father repeatedly molests his teenaged daughter. When the younger brother spies on them, he blames the sister as a slut for most of the movie until the two finally join forces to expose the father after he molests and seriously injures his newborn second daughter.
In Savage Grace
(2007) a supposedly historically accurate account of an actual murder case, a mother (Julianne Moore) has sex with her son (Eddie Redmayne)Daniel and Ana
(2009) portrays a brother in his teens and sister in her early 20s who are very close. Only a few days before her planned wedding, the two are kidnapped and forced at gunpoint to have intercourse while a criminal gang videotapes the incest scene to sell to private collectors (apparently this really happens in some Latin and South American countries). Afterwards, both siblings are severly traumatized and tell no one what happened, but the brother finally acts out his frustrations in an unexpected way -- he rapes his sister again.
I have actually seen all the movies I have mentioned, and I'm sure there are more, but that's all I can think of off the top of my head. I could probably name innumerable porn movies too, but I don't think that's worth the effort. There's no end to the porn titles that would qualify.