Two kids, one good, one not so good, would you?

He obviously gets away with this kind of behavior at home. However, when he is in your care, you are in charge no matter what. He has to listen to you because he is the child and you are the adult. Stop allowing this behavior. I've done a ton of babysitting over 30 yrs plus any child that is here playing with my own kids...they are in my care when I am babysitting or they are visiting my home. They have to follow my rules...I make it clear to every child, regardless of what they get away with with teir own parents, that when they are in my care, they follow my rules. They behave themselves because they know I won't stand for anything less. You have to be firm with your nephew and let him know what you expect of him.