[Z]

Corner the guy on his own and speak to him but I wouldn't resort to violence 'cause there's every chance you'll get done for assault. I feel sorry for your sister, and for you having this dilemma though. Bullying sucks. People don't even realise a lot of the time the effects of what they say and do to another person and just how damaging they can be. Just make sure that your little sister knows she doesn't deserve any of this and try to be her big brother as best you can.