I had that same problem with 2 nephews. One day we were outside and the younger one was hitting the older one with a stick, the parents kept telling the younger one to stop but of course he kept doing it - I think the general opinion is the older one is more mature and therefore should be able to handle it???
Anyway, I finally took the stick away from the little monster and gave him my "look" and plenty of grief. He behaved around me from then on, of course he was still a brat but oddly enough, he turned out to be a super adult and he and his brother get along well. I think, at some point, the parents wised up.