Taboo IV: The Younger Generation (1985)

Dr. Lodge specializes in inbreeding and holds group therapy class which includes Mrs. Joyce McBride. His two girls, Naomi and Robin, come home to find their family torn apart after the doctor catches his wife with his brother. The family splits, leaving Robin to stay with daddy and Naomi to live with mom and uncle Billy. Naomi is hot for Billy and her desire builds as she watches her mom and uncle. Meanwhile, knowing about the inbreeding group, Robin talks Junior McBride into seducing his mom so she can watch, leading to her own urges of inbreeding.