Need to vent about my mother

My mother was a teen mom, well 18 but still, and my father was never any help didn't want me, didn't want her unless she got rid of me etc... So she had my aunts helping her after I was born so she could continue school, sleep and whatnot. Also my grandma. See I'm 20, and my fiancé is 24 we moved in with my mom when I got hit in our vehicle and it was totaled, she helped us save for our new car, stuff for the baby. And I love my mom don't get me wrong, she wasn't a very loving mother while I was growing up I feel like she resented me for my dad not wanting her. So in turn when I was 4 she got with my stepdad who physically, emotionally and mentally abused me for 15 years. He still is a bitch to me. He's made it HELL living with my mother from locking us out of the house purposely, talking shit to my mom everyday for no real reason we just don't like each other and he tries to convince my mother in using her when in reality there's no way I use her. She put him before me my entire life and now she has a guilty conscious he has even stated that when we all argue, it's been a very hard 2 months here. My fiancé got into it with him which made it WORSE I didn't think it was possible. He's started with telling my mother we better not touch his food, he cuts off the electricity when he leaves. It's been like this my entire life sadly he's psychotic. Truly he is. And now that we have our car and I'm due anytime this month my mother is guilt tripping into staying with her after he's born, ONLY for a week. So she can "help me" but I can't, I really do not want too. It's bad enough the amount of stress my stepdad has put me in during my pregnancy but to put my child my newborn child through this nonsense is bullshit and I feel like my mother doesn't see it that way. She expects me to sleep in the loft with her and the baby and my fiancé stay in our room. I know she means well. Honestly I do but my stepdad is just going to be even more awful when our son comes and my mom denies it saying my grandma is going to be here and he won't act like that in front of her. Another thing, it's going to be xmas break soon and I have 5 younger siblings 13,11,9,6,3 are the kids ages and the 9,6,3 year old are crazy, loud and do not listen very well plus they watch tv in the den from the crack of dawn to midnight, I'm in the loft which is  diagonal to the den, no walls no doors. All the noise. I told my mom I will get irritated with them being loud and she's saying she knows more about babies and getting defensive. Every time I try to tell her we are not staying a week after he's born she flips out calling me ungrateful. It's so stressful, and I don't want my little one exposed to my dis functional ass stepdad. Any advice? We've already moved ours and the babies things out into our temp home but my mother is just, making it very hard