I'm going to move in with my gf and leave my mother and mentally retarded brother alone
Why would you want to leave your mom and brother?
I can't handle them 80% of the time at this point.
I understand your frustrations, but your mother may not be able to take care of your brother if you move out. It's not ethical to leave your brother in need in order to live with your girlfriend.
I'm only 19. As long as my brother doesn't move out, I will take your advice.
That is thoughtful of you to come around. Try to consider everyone's condition and how you can help them. It is good you are growing up and leaving the house, but first try to arrange it so they can receive assistance even after you've left.