My boyfriends hostile mother

My boyfriend and I have been dating almost 8 months. We're moving in together in a couple of weeks into an apartment on the third floor of his mother and step fathers 3 family home. This is the first time I'm moving out of my parents house- and I am the last one to leave them. So I think it's normal for me to be emotional leaving my parents- especially because my father recently had a heart attack and I'm worried about him. 
Mother's Day is obviously Sunday. I didn't even ask my boyfriend to leave his family to be with mine- I just assumed we were doing separate things with our own families. So I told my boyfriend I was making reservations for Sunday lunch with my parents and brother. He then told me dinner was at his parents house for 3:30pm. I told him that I wasn't originally planning on going over that soon because I'd like to spend time with my mother on Mother's Day. 
He then proceeds to tell me that his mother stated that "if I want to be part of the family I will be at dinner for 3:30 with them and how I'm disrespectful of I don't go over." Along with other comments about me not coming to her house Easter because I had a 102.5 degree fever
This then made my boyfriend and I get in an argument because his mother is absolutely bat shit crazy with some things and it comes out of no where. "Bat shit crazy" being the words of my boyfriend. But he wants to make her happy and doesn't want hostility between me and the family so he's half on my side but never fully. 
Anyways- I stood my ground and told him I'd be over aroma be 4:30 because I'm not going. To rush through spending time with my mother to go over his mothers. 
Anyone else deal with a hostile mother-in-law?!
We're not even married!! I'm honestly scared of when we do get married because I have a feeling things will only get worse.