How can I put in writing that if something happens to me and my fiancee, who my child can live with?

I don't like my fiance's parents, nor do I like the way that they treat their children especially my fiancee. Even though he is a grown man, his dad still yells at him and is very insulting. Sometimes he even does this in-front of my son. I have witnessed and observed the way these people are not very good parents, and my fiancee tells me stories about how they treated him as a child. I want to know that if something ever happens to us, how can I make sure that they won't have custody of my son. I would rather my older brother and his wife to take care of him. Do I have to get a lawyer, put it in writing, or what else can I do? This is a really big deal for me.