I would love to be a size 12!!!! My biological mother was like that. I was so stressed out going over to her house one Christmas with a new boyfriend. Everyone was telling me to calm down, but when I walked in the door, before I could even introduce her she said "You've gained weight". Lovely. Then at my brothers wedding I was in the bathroom and I overheard her say to some girl she was consoling for some reason "Don't I have two big fat daughters out here?" of course, my sister and I had just had babies.
So, I totally understand where you are coming from. I just wouldn't have her over for any more family get togethers. If you do, I would make it a casual one where you wouldn't have to be in the same room.