Reddit, my family member is berated by my sister constantly. How do I present this to my parents?

I'm 30 and my nephew is 23. We're like brothers and I don't get to see him often - he just graduated from college and is now situated for a career in the military. Every Christmas (and get together in general), our entire family stays with my parents and needless to say we're comfy cozy. Every year his mother (my half-sister) instigates pointless drama with him, whether it's nitpicking that he's watching TV or enjoying himself to a videogame. She's now to the point where she'll get in his face and yell at him. Last year she grabbed him by the shirt and yelled at him but he retaliated for the first time by grabbing her shirt and telling her not to touch him again. He's now staying at my house and hasn't visited with the family because he's stressed out and can't handle her.

The kicker is that she JUST told him that he has a different biological father. Coupled with the incessant bullying from my sister and the inability to enjoy himself in the only place he finds home, he's becoming really sad and depressed.

My mother is worried sick that if she confronts my sister, she's not going to allow visits from her side of the family, including my nieces and nephews. She has credible reason for this worry because in 2002, my sister fought with my mother over the use of a vehicle, and when she wasn't permitted to use it, she took away the kids and didn't allow my family to see them for over 2 years.

Given all this drama, I'm going to present this to my parents, but I'm scared in the approach. I've been in MANY conflicts with my half-sister before and she's a bully. She's quick to yell, resist, and flee before compromising. If I approach her myself, she rarely listens and doesn't view me from an authoritative position. If I approach my parents, they do hold power, but are afraid of losing the kids again.

Thanks for reading this. I'm thankful for any help you can give Reddit.