What would you do?

So I have a 5 year old son, who is vaccinated. My husband and I did tons of research, watched documentaries, and got lots of patience and advice from our amazing pediatrician. I'm now 19 weeks pregnant with our second baby, and I just found out yesterday that my brother (only sibling) and his wife have decided that they will not be vaccinating their 5 month old baby...ever. I want our kids to be super close, but I've heard so many mixed stories from both sides. How would you approach this situation??