These aren't necessarily movies about incest, but movies where incest has come up in one of the relationships depicted in the movie. Currently the practice of incest is banned in almost all civilized countries, with punishments in some being worse than others. Though some cultures encourage the practice among cousins, the societal norm is blood relation means no invitation... for the making of the sex, that is (I apologize ahead of time).
Now, while these aren't incest movies, they do happen to have overtly incestuous themes. Sometimes the incest is played out like a joke, but most of the time it is in an uncomfortable way. Whether it was for revenge, love, or just because they were ignorant of their connections, the incest in these movies is usually hotter than it should be.
FYI this site also has a list of books about incest.
http://www.ranker.com/list/13-steamiest-incestuous-relationships-in-film/roger-nackerman,
Back to the Future
Back to the Future is one of the greatest science fiction films of all time, but it's all really a cover for making a movie about a mother having the wrong kind of feelings for her son.
Marty McFly inadvertently goes back to 1955 and prevents his parents from meeting, at which point his mother becomes infatuated with him. Not only does she come on to her son, but they even go "parking" during the "Enchantment Under the Sea" dance.
Oddly (and very creepily) a segment of the Internet populace has made videos in support of the relationship. Weird.
Back to the Future trivia: When Robert Zemeckis was trying to sell the idea of this film, one of the companies he approached was Disney, who turned it down because they thought that the story of a mother falling in love with her son (albeit by a twist of time travel) was too risqué for a film under their banner. In fact, Disney was the only company to think the first was risqué. All other companies said that the film was not risqué enough, compared to other teen comedies at the time (e.g. Fast Times at Ridgemont High, Revenge of the Nerds, etc.).
The video features a collection of the Marty and Lorraine scenes that ask, "What would you do?"
Cruel Intentions 2
The first Cruel Intentions had plenty of implied incestuous undertones between step-siblings played by Sarah Michelle Gellar and Ryan Phillipe, but who could blame them; they're not blood-related and they're both hot as hell. I can admit that as a man, and so should you.
The straight-to-video sequel (sadly, this was the first of many), which featured a pre-oscar Amy Adams, one-upped the original by not only featuring sister/sister incest, but they took a page out of "every porn ever's" book and did it with real life twins. Click here for the NSFW video of the incestuous action, and some of the worst ham acting in film history. These people moved to LA to do this...
EuroTrip
Michelle Trachtenberg is insanely hot. After years and years mired in little sister and teen roles, she finally got to show off her sex appeal in Eurotrip as the also insanely hot character "Jenny."
As four friends made their way across Europe, she and her brother had a number of misadventures, possibly the worst of them, though, is featured in the scene to the left where twins Jenny and Jamie end up getting so drunk on Absinthe they have no idea who they're making out with.
The incestuous liaison happens at the 3:30 mark and is... uh... yeah...
Sleepwalkers
What is it with cats and incest?
The cat people in this Stephen King adaptation (featuring King himself as a coroner) are mother and son, but also creepily enough, lover and other-lover. They make "love" to the song "Sleep Walk," by Santo and Johnny, forever ruining a classic that makes you want ice cream, sunny days, and women in '60s bikinis.
The jealous rage the mother has toward the girl that her son is seducing (to essentially turn into a meal since they're energy vampires) is both hilarious and unsettlingly creepy.
Star Wars Episode V: The Empire Strikes Back
You knew this was coming. The most famous case of incestuous relations in pop culture. Luke Skywalker was enamored with the idea of saving a beautiful princess and considered himself a rival of Han Solo's for Princess Leia's affections for most of the original Star Wars trilogy. (Check College Humor's edit to see just what Han was thinking.)
Long before it is revealed to them that they are in fact brother and sister, the most famous twins in a galaxy far, far away lock lips in the scene to the left, complete with reaction shots.
If you think that's creepy, in 1978 there was a sequel novel to the original Star Wars (before the rest of the trilogy came out) called Splinter of the Mind's Eye. In fine piece of somehow-published slash fiction, Luke and Leia are stranded on a swampy planet alone. The amount of sexual tension in the book is palpable, and there is even a scene where they playfully wrestle in the mud and nearly kiss. The book was meant to be a low budget sequel in case the first film bombed. I think they should've filmed it anyway.
The Blue Lagoon
The Blue Lagoon is a firestorm of controversy, from underage nudity (there isn't any, it's a body double) to the incestuous relationship that develops between the two (related) castaways, the film manages to upset every moral group's sensibilities.
While it isn't fully clarified in the film, the two main characters are either cousins (which is kiiiind of okay for the time period it takes place in, but still, it's rather grody) or brother and sister (not cool).
Either way, the two are definitely related, and the baby that they have together in the film is at a high risk for genetic defect, which once again, makes this come to mind.
The Devil's Advocate
Keanu Reeves plays hot shot lawyer Kevin Lomax, who Al Pacino has big plans for at his very high profile law firm in New York.
Thing is, Al Pacino is the Devil, and he's really trying to get Keanu to join the family business. Yup, Keanu is the son of the devil. Keanu is married to Charlize Theron in the film, but that won't do at all, because daddy has someone special in mind for our Mr. Reeves. Connie Nielsen plays Christabella, a temptress that has Kevin Lomax (Keanu) thinking, "Charliz-eh who?"
In the scene to the left, the final scene in the movie (spoiler alert for those who hate themselves enough to watch this and attribute absolutely no value to their time enough so that watching The Devil's Advocate seems like a good idea), Al Pacino plays matchmaker between his son, Kevin Lomax, and Christabella, his daughter.
And it's at this point that Keanu and Connie get into some hot and heavy action as brother and sister, with Keanu even uttering the line, "I've wanted you since the day we met," which for MOST brother/sister couples, is the moment the younger one was born.
The Dreamers
If you've ever visited one of those celebrity Mr. Skin type sites, this is that one movie that seems to be almost exclusively about two dudes having sex with a really really hot girl. Kind of like Weird Science, only they didn't have to literally build her.
Set in 1960's Paris, the movie is about an American who gets close to two people he at first thinks are a couple and then is told they are actually twin siblings.
Over time, the relationship between all three blurs into a weird sort of communal love, but the incestuous boundary is definitely crossed. Eva Green stars in a pre-bond girl role where she makes love on numerous occasions to a guy while her brother watches, pleasures himself, and then even "has relations" with the other guy himself.
What happens when their parents finally come home and see what has become of their kids?
The parents immediately grab their things and abandon them, which goes to show why they probably turned out the way they did. Eva Green's character then has a moment of clarity and tries to kill the three lovers for what they have become. Here's a NOT SAFE FOR WORK link to the first love scene in the film, just to give you an idea of how disturbing it is Dreamers "love" scene. Seriously, it's NSFW, but hey, it's a European "art film," so it's okay.
The House of Yes
The epitome of the incest movie, Parker Posey (from all the Christopher Guest movies and that half-assed Superman reboot) plays Jackie O, the twin sister of Marty (what is it with twins?)
Marty has returned home for Thanksgiving and also to introduce his family to his fiancee. Jackie is more than upset at the news, considering she's in love with her (twin) brother, and they've been sleeping together for years.
The video to the left is them breaking the news of their incestuous affair to their brother, played by Freddie Prinze, Jr. who, hey look at that, still does some acting sometimes.
The Lion King
The Lion King is the story of lion cub, Simba, who witnesses his father Mufasa's death and is guilted into exile by his evil uncle, Scar. While in exile, he is reunited with a cub from his pride named Nala. Nala and Simba fall in love and make with the love on their first date, because Nala was raised, literally, in the wild.
If you've ever watched Animal Planet or a documentary on Discovery about lion prides, you know that a pride is made up of one male lion and a bunch of hungry, controlling, horny lionesses. The male is the only one who sleeps with these lionesses and is therefore the father of any cubs in the pride. Based on this knowledge of how the animal kingdom works, we can assume Mufasa is the baby daddy of both Simba and Nala; making both of them half-siblings.
Thanks to Disney (and the story they originally stole the Lion King from), you can now never unlearn this.
To make matters worse, in The Lion King sequel, the pair have had a cub of their own who falls in love with Scar's son. Remember, Scar is Simba's uncle, meaning even in the second generation, The Lion King kept it in the family.
This comes to mind when taking all of this in and was intentionally excluded because c'mon, seriously.
Not really related, but you need to see this mash-up of The Lion King with the audio of Dark Knight Rises. Simply amazing.