I agree - and I'd be in tears as well. I do think Cain needs to get involved now, he needs to make his mother understand he can't treat you, his future wife, this way, and especially not in your home.
Is she insecure and afraid you're going to take Cain away or something? I know some MILs get like that. Maybe Cain can make her understand that you are important to him, but so is she. There's room for both ladies in his life!
And honestly, if she doesn't change at all, maybe he needs to talk to her about alternate living arrangements. I guess it is only 2 more mos. - but still, it's not fair to you.