Need advice please!!!!

My brother just moved in with us as he is getting divorced. He has his kids on the weekends. Their mother is an anti-Vaxxer. That’s her choice. I’m pro vaccination. I’m not sure what to do when my baby comes I’m 28+2. I don’t want their unvaccinated kids around my newborn but i can’t tell my brother he can’t have his kids here. I just found out today they have no vaccines and I’m not sure how to approach this or what to do!!! Any advice??? I’m going to speak to my ob at my next appointment but it’s not for another 2 weeks as I just had one today (obviously before I found out)