You're into some very deep and longstanding conflicts here AF.
In the present, your brother is about as much of a protector as he is a high school football star. By whatever twisted consequence of fate, karma, or disease, he's come to identify with your abuser, your father, as an adult. I'm afraid that there's no going back for him without serious, intensive and long-term help. I can also tell you that what you've described is not a rare occurrence within families, without going into unnecessary and potentially harmful detail. That's a conversation or conversations for you and your therapist.
For as long as you continue to engage your brother, in any way, he will continue to play the role of abuser. What I'm seeing is that you've been mourning a loss that's already occurred, rather than a loss that's in the offing were you to cease having contact with your brother. Incomplete and/or unresolved grief can be a tremendous obstacle in terms of getting sober and living a meaningful life, and can be a trigger for relapsing that's on-call, 24/7.