Sin of Innocence (1986)
When David, a widower with a teenaged son, marries Vicki, who is divorced and has a teenaged daughter, it's sure to be a happy family. But the kids, Tim and Jenny, become romanticly attracted to each other. Which makes David and Vicki VERY uncomfortable, and Vicki's ex-husband, Andy, furious. It's no longer a happy family in this household.