The Unseen (1980)
http://www.imdb.com/title/tt0083261/
I did not write the following review, but it kinda sums it up..
The plot is not a new one: Three sexy chicks stay at a place where a brother and sister (who had incestuous relations) keep their deranged man-child in the basement. He, of course, kills off the girls one by one. Most of the movie is sleep inducing, but worth the watch. I was impressed with Sydney Lassick's performance as the seemingly kind, yet twisted brother/father to the murderous offspring. He definitely gave the part his all. Love the clothespins on his face routine. Beautiful. Then there is the mentally retarded guy in the cellar, amazingly played by Stephen Furst. He is both disturbing and hilarious with his mentally retarded mannerisms and groaning. The guy sticks a teddy bear in his underwear and blithers like a loon. Good lord, what a pure genius performance! Every time he's on the screen is a pleasure to watch. Get this movie just to see these two guys ham it up. If the plot begins to put you to sleep, fast forward until either one of these guys are on the screen. You won't be disappointed.