How to stop an uncontrollable kid at my parent's anniversary party?

He is the brother of a guy who used to be uncontrollable at parties as well but is now fine. This brother is a million times more crazy; to the point where if he gets beat up by someone he will keep annoying them to no end without caring about his own health; maybe he'll cry for five minutes but acts like its fun after two minutes; and i mean after hazing. I don't think conditioning would even help unless it was done for more than a week or something. I'm really not trying to be mean; I love kids and babysit and even visit my neighbors sometimes. I'm just worried he's going to freak out and throw tantrums the day of my parents anniversary which will be held at a fancy place and the whole thing is going to be like a wedding and I'm hosting the party. His parents expect me to take care of him but sometimes they tell me to leave him alone because disciplining doesn't help. He annoys my brother to extremes throwing tantrums to the point where his parents are encouraging my brother to beat the **** out of him......What should I do? I need to be prepared...