Good grief. No wonder that girl has the ... nerve... to attack a 6' victim... her brother has her back and isn't in the least worried about beating a girl... makes me wonder just what's going on at home to put the idea that that is FINE into his head.
Have the police, CPS, anything like that investigated to find out WHY these kids are acting this way?? It could just be rebellion, absolutely. But it could also be drugs, from peers. Or it could be some major problems, even abuse, at home. No telling unless someone goes in and snoops... for their sake, AND for every other student's sake I think in this instance some intervention is warranted.
I hear you though... it's a fine line between defending yourself and looking for a fight... when to walk away, when you can't. I won't stand for someone hitting me, or mine, but I also don't bring fists into an argument... not with the laws as they are... when the victim is punished as harshly as the attacker... nope. I think before wading in because if I do anything but curl in a ball and take it I'm going to jail. Just make sure she learns that part of the lesson... is aware of what will happen if she can't/won't walk away... and then you gotta leave the choice to her. Just be sure that when she's in the right, when she didn't start it and COULD NOT walk away, that you support her not laying down and just taking the boot to the face. But, that's where that line comes in... if you weren't there it's hard to tell whether she could have walked away or not... trust IS a bugger isn't it.