It's normal for siblings to fight from time to time. But when fighting gets to the point that one child is becoming emotionally or physically victimized, it needs to stop. Repeated hitting, biting, or "torturing" behaviors (for example, incessant tickling, teasing, or belittling) are forms of sibling abuse and justification for you to step in. If you can't stop the violence yourself, talk to your child's pediatrician or a mental health provider to get immediate help.