cousins in same class

Hello,

I am in need of some advice, my 2nd grade son is in the same class as my neice, it's a small private school, there are 10 students total in the class, my son and neice annoy each other often, yesterday an innocent student got hit with a lunch box intended for my son. Nic (my son) started the mess by saying she was "mean", and it fueled until she through the lunch box. I have tried over and over again on my part to strees to my son that he can't alaways say what he thinks! I've also always been the one to enforce that the 2 "just get along". I know she starts it at times and I know he feeds on it too, and he could have gotten angry enough to toss that lunch box as well. I am constantly talking to him about this, to th epoint where I think he has tuned me out when her name comes up, he KNOWS better, but my brothers family are great "blmaers" never their fault, and I feel as if I'm the only one who wants control over this situation. I adore the school, and my brother has every intention of pulling his daughter out before middle school, I do not. I don't want to leave the school, and I've talked witht hte principal and teacher, and they just keep saying we've got to teach them to just get along. They were referred to as water & oil.

Does anyone else out there have cousins int he same class or was in a class with a cousin?

Any input would be greatly appreciated,as I think I'm on my own with this, not confident I can speak to my brother about this problem, without it being "my fault" he is the way he is!