How would you handle sibling abuse between your children?

I'm at such a loss. My 13 year old is so mean to my 8 year old. If anyone else treated my child that way, I would get rid of that person or call the police. But the abuser is my other child! I don't know what to do. Most of the abuse is emotional. It is relentless and cruel. He makes fun of literally everything his brother likes, laughs at him any time anything negative happens to him, ignores him when he speaks, or makes a noise every time he starts to talk, calls him fat, rejects his attempts to be affectionate. When my little one becomes so frustrated that he finally lashes out, his big brother retaliates, once leaving red marks on his back from his fists. I have tried traditional taking away privileges and grounding, but the incidences are so frequent (every few minutes) that I can't keep up like that. I keep getting calls from school and church to come pick up my son because he has broken down crying or become violent while talking about his big brother. 13 year old is in therapy.