Playing with Fire (1983)

Incest, by the best in X-rated films, Kirdy Stevens. A divorced man falls in love and then marries a divorced woman in the first third of the film. The man, Danny, has one son, Les, and the woman, Cathy, has two daughters. However, the two sets of kids haven't ever seen each other. So when every mets at the dinner table, Dan discovers he has had an affair with Cathy's older daughter.