Siblings are notorious for creating chaos and havoc in their brothers' or sisters' lives. If you're talking about physical abuse, and you have brought this to mom and dad's attention and nothing is being done, I would suggest video taping the abuse and then showing them what's going on. For any other advice, please provide some additional details - is the sibling older or younger? Brother or sister? What's going on? Etc. And I'll try to help.
My older sister used to bully me, she used to beat the tar out of me whenever my parents were not around. All my mother would say was "one day she'll be bigger than you", my sister never got into trouble for this and as an adult I really told my mother how much it hurt me that she didn't intervene.
PS. That day did come, one day I did realize I was bigger than her and I sent her flying across the room. To this day we are barely on speaking terms.