Taboo IV: The Younger Generation (1985)

Dr. Jeremy is a popular sex therapist and runs an incest support group and his patients include Joyce and Barbara, who have incest relationship with their own sons and only appears in brief flashbacks. He's also deals with his own issues of dysfunction. He is a very strict father and has sent his two daughters to boarding school. The eldest daughter, Naomi is caught having sex in her dorm and is kicked out of school. When the two sisters return home, he discovers that his wife Alice has had an affair with his brother Billy and throws her out angrily. Naomi chooses to live with her mother and uncle -later revealed to be her biological father, while Robin chooses to stay with her father. Naomi seduces her uncle/father and has sex with him, while Robin develops a yearning for her father. As family ties fray, bedroom hopping begins, testing these bonds to the fullest.