Contact the police and file a criminal complaint for contributing to the delinquency of a minor against your niece. You might also be able to have juvenile court restrict your son legally from being around your niece if you have him charged with being unruly/delinquent. Those are two options you can do legally.The major issue here is that your son refuses to listen and that means you need to deal with him and punish his behavior. He is the one for whom you and your ex-wife are legally responsible. Hence, you need to crack down on his behavior. Some suggestions would be to remove him from the Garden of Eden -- everything is removed from his bedrooms except basic necessities -- mattress, bedframe, t-shirt, jeans, underwear. All games, electronics and what not go. He can earn them back. He should also be restricted to school and home. IN addition, civilly, you could sue your niece for the injuries your son sustained in the car accident. Those are some options you have.