The Addams Values (1993)

Gomez and Morticia welcome their new baby, Pubert, to the horror of siblings Wednesday and Pugsley, who try to eliminate him. To help manage the growing family, they hire nanny Debbie Jellinsky (Joan Cusack), who secretly is a black widow planning to marry and kill Uncle Fester for his fortune. The Addams children are sent to a summer camp, Camp Chippewa, while Debbie successfully marries Fester and tries to change him, leading to a confrontation where Wednesday saves the family by exposing Debbie's murderous past.