Maybe there is nothing you can do physically, but I have said this already.

You don't jump from trying to stop him yourself to asking the OSPCA to step in. You tell the authority figure in the house... your *parents*.

Again, abuse is not okay. Just the fact that you said that your brother is turning her aggressive shows that she's enduring emotional abuse. An aggressive dog with children in the house is a case for disaster. IMO, this is just asking for the AC to step in. If you really love this dog, rehome her.