suffocated

Need yr advise. I'm a new mom , gave birth last month. I'm extremely happy to welcome a newborn in my life although at this age most would go for career (I'm in 24). At the moment , I'm staying with my fiance and his parents. Been with him for 6 years now and will be getting married in a few months time. The issue here is i hate his mom. My fiance is the only son and sole breadwinner of his family and i actually started to hate his mom for the past 4 years due to her casually blaming me for something that went missing. His parents are elderly (mom at 60s and his dad at 70s) and sometimes , his mom likes to manipulate others to get what she wants. It's the little thing she does that causes this annoyance in me to trigger. I extremely loathe her behavior. I stayed with fiance ever since we started dating and I've been cooking , cleaning for the family. There was once she lashed out saying im a good for nothing who is staying with them for free. Hello? Then who does all the housework? All she does is eat , shit , sleep. And dont start with hygiene. She sometimes pee in a pail because one of us are in the toilet (staying 2 room rental) and will forget to wash it or will simply say later will wash. All this little things are a trigger for me cause now with my newborn and the marriage coming,i don't know if i can stay with them or put up with her behavior. When my lo cries, she will come and kpo and the word that always comes out is "oii why you cry..." like that's annoying imagine every single time hearing that phrase when yr lo cries.. i lashed out the other day when she came in to check and honestly, what can she do if my lo cries? She cant even take care of herself and that is why i cant entrust them to take care or look after my son even for a second. I just hate it when she comes to check cause with lo crying and her irritating voice and phrase, it adds on to my lo crying. I cant expect to move out cause like i said , fiance is the only son and he's supporting them financially. What other options i have? I feel so suffocated here . My family are totally out of the picture as i do not wish for them to get involve due to certain issues... What are yr advises for me to handle this stress im facing???