SO wants to move in with MIL pt.2

So I talked to my SO and told him I wasn’t ready to move out of our house and I don’t want to go from living on our own to moving back in with his family. His mother is stressful and toxic and I do not trust her for anything. You guys commented and said to just tell him I wasn’t moving and he could move out on his own. Well he plans to do exactly that. We have another baby on the way im a SAHM if he moves out he will get split custody of our babies and I will have to go back to work. Which I dont have a problem with working. But I absolutely have a problem with my babies being left alone with this woman. I do not trust her she is not a safe driver or in my opinion a good parent. She’s constantly driving over the speed limit and angrily driving. Her daughter who is a minor does drugs and she doesn’t do anything about it. I don’t want my kids growing up in that situation with her. I dont know what to do.

So I’m trying to stay anonymous just in case someone I know is on here. But he’s wanting to move in with her because he wants to save money for a house. Which I understand I want to buy a house too. But he’s wanting to stay there for a year and I’ve lived with her before and it was not easy and I do not like being that close with her. I dont want to raise my babies with her and her daughter there. I don’t know what applying for primary custody means? Or how I would go about this