Sin of Innocence (1986)

Synopsis
While we're tempted to label the TV-movie Sin of Innocence as Brady Bunch: The Lost Episode, the film transcends all kidding with its intelligent, tasteful approach to its story material. Dermot Mulroney plays a teenager whose widowed father (Bill Bixby) marries a divorcee (Dee Wallace Stone). Suddenly Mulroney inherits a stepsister (Megan Follows), a girl his own age. What should have been an uncomplicated setup becomes problematic when stepbrother and stepsister fall in love with each other. Sin of Innocence comes to a logical and satisfying conclusion with the two young people solving the dilemma themselves, without the self-serving "assistance" of their anguished parents. ~ Hal Erickson, All Movie Guide