I need help with a problem with my neighbor that lives in the apartment above me. I have 3 children 14, 10 & 6

I am going through a divorce. I live in a duplex apartment. I have lived here for 3 years. My upstairs neighbor has been here for a little over 1 year. The problem is that her son and my son are in constant conflict. The mother is never around when her son is doing something but she is always around when my son is the "bully" when in reality for the most part he is retaliating. I know that neither child is totally faultless. Last night she was outside yelling at my child for saying that her sons Jordans (sneakers) were fake...(my thought is who cares). My son says that he didnt say it but then her son calls my sons sneakers fake (and thats ok). Her son has also given two death threats to my son. The first one on "myspace" two months ago and last night he says "Im gonna blow your brains out". The mother then took her son to the side. They also think I am stuck up because I dont sit outside and drink with them. I dont have time and have other priorities.