Living with MIL

So my mother and sister in law are moving in with my fiancé and I in a few days. My fiancé told his mother that she could stay with us if she didn’t find a place to go when the lease to her apartment was up. She immediately just said “oh I’ll just stay with you” 🙄 my fiancé didn’t ask me if I was okay with it before doing so, it was too late to say no. My baby is due in June and they’re planning on staying for about 6 months. This will be my first baby and I really just want it to be the 3 of us. Has anyone dealt with anything similar? Any tips to not go absolutely insane? I haven’t lived with anyone other than my fiancé in a long time and I’m not looking forward to the switch 😭😭