I've decided that my fiance's brother and newborn can't live with us and I'm going to tell her.
What's your justification for them not living with you?
Because my fiance's brother has not yet visited us nor paid us a visit since the new baby was born
While I get that can be hurtful, you have to know that a new baby is very time consuming and stressful. They have no obligation to visit you when they need to take care of their little one.
I'm most upset that I won't get my quiet time.
It seems your fiance's brother and the newborn need a place to stay and the right thing to do is to help them out and welcome them into your home. Believe me, I love my quiet time too, but sometimes you have to make sacrifices and ultimately this one is worth it.