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 (or incest films), 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.