Mother in Law Problems (advise)

My husband and I keep having issues due to my mother in law well kinda let me explain. He is buying a house with his mother because she is getting divorced. According to him its going to be our home and she is living with us but when I ask her the way she talk about it it makes it seem like its going to be her home and we are moving in with us. I have been pressuring my hubs to move out to a bigger home as I am pregnant with our second but i dont think moving in with his mother is the greatest idea as we both clash constantly due to both of our personalities. Today they are signing the contract and before he left I told him that i didn't want to live with his mother he said that i pressure him too much that i can't demand things (I'm a stay at home mom) and that his mother needs a place and he has to help her. He left without saying goodbye and hasn't answered the phone. Idk what to do. Idk if I should wait and see how it all works out or just idk. Why do mothers in law have to be so dependent of their sons