How do you deal with rude, bratty cousins?

So my sons two cousins that are close to him in age are brats. They are both 3.5, and he is 2. The little girl isnt so bad, but the boy is the biggest brat. Hell do things on purpose to hurt him, get him in trouble, call him names, etc. My son is allergic to peanuts and I specifically ask him not to give him anything because I have to read the label first and he does anyway. Then he'll say "this doesnt have peanuts it's a lolipop" or whatever. Hes super annoying. The mom is way too lenient with him and never follows through with punishments. He knows if he cries long enough he'll get whatever he wants.

My boyfriends dad always says he wants our son to be close like those two are with each other, but we would rather keep our distance for now. I know it's not his fault, and the problem lies with his mom, but shes a lazy parent. Anybody else have this problem? Any advice?

**Edit: Ohh yes I have yelled at him about the food situation. He does not listen. It makes my blood boil to be honest.