Tantrums

My nephew is almost 4 years old. He's reached a point in his life where everything is MINE MINE MINE! Even if it's a toy that's shared with his sister, it's MINE! We're trying to teach him to share, but he has no desire to and it's difficult. If he doesn't get his way he screams and yells, throws temper tantrums and gets so worked up that he'll even make himself sick. But we don't give in. If you reward behaviour like that, then he'll grow up thinking that's how he'll always get his way.