Help on a crazy Grandma?

Ok so here's the deal. I have been dating my boyfriend Matt for a year and 4-5 months and his grandmother is making me soooo mad! His mom to! Matt's grand has Dementia and Alzheimers, but we all know that's only a part of it. I grew up around people with those conditions and they where much easier to deal with and they where in the worse stages! His grandmother has selective memory and constantly fights with Matt and even his mom because she favors his cousin and his dead for 8 years aunt. She constantly has to be in control and can be very childish. She has gone in Matt's room and stolen stuff from him. I leave some clothes there cause I'm there alot and she stole some of my clothes and jewelry! Even makeup! Matt and his mom have told its hi room and he needs privacy but she goes in when we're gone anyway and if we lock it to keep her out she freaks out and says he's a little **** and can't lock His room in "her" house. She freaks out on him when he tries to be nice to her to. I let it slide until today. It was my grandma's birthday and someone "shook" her up today so I said come get me and we'll go to lunch. Matt was going to be back soon (he went out) so I waited for about 30 minutes. (We have to basically have to babysit her) My gram was getting kind of upset so we left (she CANT STAND his grandmother). I asked her that if Matt wasn't back and if friends of his showed up to not let them in his room because they go through his stuff. I realise that I went over my bounds a little bit but she seemed ok. When I got back Matt was here and everything seemed fine. I asked her if I could borrow a crotchet hook and asked how she was after I left. She said fine but angry. I looked at her and she snapped at me to not tell her who she could let into "HER" house and wear. She said to forget about it though even though I know she is going to complain to Matt's mom and lie like always. His mom never really tries to get her help for her issues! HELP me please!!!!