Name of a 80s movie

I saw this movie as a kid. It’s about a couple of kids, siblings probably, convinced their new, young step mom is trying to kill, I think, their dad. It’s all a bit muddy. In my vague memory it looks like an 80s or maybe early 90s film but I can’t be certain. But it was an old film. Not a horror, more of a mystery film. The audience is kept at a suspense as to whether the step mom really is wicked right until the very end.