I did not know that Little Miss Dangerous was in that movie. That was the sequel to "Theory of Relativity" right?

To set things right, yes there have been incest-themed movies (none in the U.S. of late). Read the following RAME thread for some of the classics. And of course there was the Taboo series with Kay Parker. In addition to that list, there have been some recent scenes. A scene in the Don't Tell Mommy series had Rebecca Bardoux and Gia Paloma doing a GGB with each other as mom/daughter. A scene with Trinity Maxx in White Trash Whore 18 has a scene with a few dick and their "cousin." There was a 1978 movie called Teenage Twins which featured a lez scene and a GGB with Brooke and Taylor Young, real-life twins. A check of the Euro scene will yield many others, mostly in German and French (I knew they were fucked up). There is apparently a real-life Euro series with the so-called Maniado family, but you'll have to find a Euro outlet to get it.