My Mother... My Brother... and I. (1973)

Sinopsis

Carol, a pretty teenager girl with incest fetish, talks about the (possibly imaginary) time with her brother Lance, who is too shy to have a girlfriend, their mother Nora, whose husband passed away and Steve, Carol's boyfriend. She gives sex lesson to her brother to help with her girlfriend and mother joins them when she returns from shopping. Later they have a wild orgy in bed together with Steve.