Playing with Fire (1983)

Brooke West & Randy West are a family in...

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 everybody meets at the dinner table, Dan discovers he has had an affair with Cathy's older daughter.