When one child refuses to share his toys with the other I remind him later when his brother doesn't share with him that he gets the courtesy he gives. This usually results in him sharing next time. I do make it a point to set boundaries since they share a room. Certain toys belong to a certain child and some toys are for both. Usually if it was a very special gift and one doesn't want to share I won't enforce it because truth is you can't have everything.