I'm trying to remember the name of an older horror movie, maybe the 70s-80s. Its about a family (mom, daughter, and son) who move in with their wealthy grandmother. I think the mom gets sick first, and the kids think the grandma is poisoning her. I think the boy (around 6-10 years old) gets sick next. The oldest kid is a girl in her young teens. I think there are three kids. I remember the grandmother lives in a giant mansion with a huge grass yard, (bigger than a football field). I think the children are forced to live up in the attic, or perhaps a hidden room in the enormous house. I also think the small boy had a pet mouse that might have died.

Try this:
Flowers in the Attic (1987) - IMDb

It's definitely Flowers in the attic. The mouse died because there was poison dusted on the biscuits that the children were given (on the days that they were fed at all). The poison killed the young boy, Cory, and it got his pet mouse as well. Here is a link to the trailer: Trailer: Flowers_in_the-Attic (1987) - YouTube