This is an issue that worries me with my own kids. However, I'm finding the abuse paradigm a little too simplistic to be helpful. It seems that my older two abuse each other, and both would claim that the other is the 'abuser'. They constantly harrass each other, and frankly, can be awful with each other, and it is nearly impossible to sort out whose 'fault' it is. I would love a better solution that my current one, which is when they fight, they both go to their room until they can act like civil family members again (and really, that civility is so painfully fleeting).