Sibling Rivalry (1990)

Marjorie Turner is suffocating. Her younger sister Jeanine has no ambition, and her eight-year marriage to Harry puts her in constant competition with his family (all doctors), with whom he too is competing for affections. One day she snaps while food shopping and beds a mysterious man who dies right afterward. When she forgets her wallet, a vertical-blinds salesman finds it and things really get complicated from there.