I know how you feel. You are in a horrible position. My brother is a 40 year old addict who lives with, and emotionally abuses my mother. I've struggled to know how to deal with it my entire life. (Before he started drugs at age 13, he was a major behavior problem).
I used to fight with my brother all the time, with similar results to your fights with your brother. He's not physically abusive anymore (probably because he knows I WOULD call the cops), but he is extremely verbally abusive. Having grown up with me, he knows exactly what to say to REALLY hurt me. Consequently, I am still very scared of him. In fact, the entire family is scared of him which is how he controls the situation. Anyway, my standing up to him on behalf of my mother did absolutely nothing except make me sick. I finally realized that I am no match for the demon of addiction.
I looked into doing some sort of intervention because like you, I am worried about my mother. However my therapist said it would never work because my mom is so heavily enabling him. So now I've given up. I don't have a relationship with him, and I only see my mom at my house. I'm working on my own recovery which apparently is all I can do.
My advice to you is to move out as soon as possible. I just don't know how you will stay healthy if you don't. It's too hard to see two people you love so much self destructing without getting dragged into it yourself. You can't protect your mother unless she wants protection and is willing to do the hard work necessary to learn how to detach from your brother.
Objectively speaking, you should call the cops and report your brother for punching you; it would be the healthiest thing for everyone involved. However, I understand how hard that would be because you don't want to hurt or alienate your mom. Just remember though, when your mom is asking you to protect your brother from his actions, she is doing it from a place of sickness, not love.
If you can't move out, go to Al Anon ASAP and keep posting here. You really need to focus on saving yourself right now.
Hugs to you