What movie is this called?

I forgot what this movie is called . it looks like a 1980 movie or somewhere in the 1990s. its about theses 3 kids and a wife. the mom locked thes kids in her mothers attic and she go does whatever she wants. and she and her mother feed them poison cookies to kill em. and they are all locked up in the closet only the mom and the grandmother knows. and at the end the daughter pushes the mom and i guess she died at the end. thats the only thing i remember about the movie. it has been killing me for years now.