As a parent, it can be tricky to distinguish routine sibling squabbling from unseemly behavior that requires adult intervention. My sister and I would fight like cats and dogs when we were kids, but our conflicts rarely escalated to the point where our parents needed to punish one or both of us. «Stop going into my closet and borrowing my clothes without asking» is a pretty low-stakes debate that kids should learn to work through on their own. 

There’s a difference between conflicts that don’t have a clear answer and one sibling constantly putting the other sibling down. It’s the parent’s job to help maintain standards for behavior in their house. If one kid can say whatever they want about the other kid without any consequences, it will have highly detrimental consequences for both of them in the future. Nothing good comes out of being the tormentor or the tormented.