Is she a millionaire and stored is in cash in the attic? Does she have dirt on your family members hidden in the house? Something is in there that they don’t want people to know about, or want to keep for themselves.
Pretty much you are fucked, you could always sleep in her house yourself and call the cops when the family tries to break in. Sounds like your family doesn’t trust your dad. Is there a reason he isn’t liked/trusted?