Ugh...I really hate the "mine" thing...especially from a child that old! He is old enough to have learned better...but obviously hasn't. My guess is that his parents have cultivated this in him. They probably tell him he does not have share any of his toys with his brother, that his brother cannot enter "his" room, that many things at home belong to him and him only and he does not have to share. I see this often and it really makes me cringe. Ask the parents what they are doing at home to curb this behaviour and take it from there. If they are cultivating it, it will be an uphill battle for you. I am sorry, I am tired and don't have better advice, just empathy at this point because that was one of the many issues dcb I am terminating has!!!!!!!!!!!