I watched this movie when I was younger so my memory of it has faded dramatically.. I remember it was about a couple of young boys and an older sister alone in this house.. Somehow hell comes through a gateway in the floor at the end.. One of the boys turns into a rat.. Please help I have been looking for this for a long time!

Sounds like The Gate (1987) (http://www.imdb.com/title/tt0093075/)