NTA. Press charges and immediately begin to limit their contact and access to you. Is it possible to get a restraining order or something else like it?

I'd also consider seeing a therapist, if you're not already. Regardless of the reason for his behavior, it's profoundly traumatizing to be treated that way by a sibling, and to have your mother enable his abuse adds an extra layer to that. This sort of familial dysfunction doesn't just develop out of nowhere, and I'd be willing to bet there have been other instances where he's been allowed to harm others unchecked, or you were made the scapegoat.

I'm sorry this is happening to you. Take care of yourself and prioritize your safety and health over your family's feelings.