Season of Fear (1989) / Stepmom sex

A twisting, turning summer of passion… and murder. A famous inventor marries a beautiful but much younger woman. His son becomes romantically involved with her. Fred Drummond (Ray Wise) dreams of being reunited with his grown son Mick (Michael Bowen). Fred hasn't been in Mick's life for two decades. In the meantime, he has married Sarah (Clare Wren), a beautiful woman who is many years younger than him. The reunion seems to be going well until Mick and Sarah start a whirlwind romance. If the bizarre love triangle between father, son and stepmother wasn't dangerous enough, it soon gives birth to an even more complex and deadly plot.