My stepmother is an alien (1988)

Trying to rescue her home planet from destruction, a gorgeous extraterrestrial named Celeste arrives on Earth and begins her scientific research. She woos quirky scientist Dr. Steve Mills, a widower with a young daughter. Before long, Celeste finds herself in love with Steve and her new life on Earth, where she experiences true intimacy for the first time. But when she loses sight of her mission, she begins to question where she belongs.