My younger brother is purposely letting toxic friends control and damage his life

I (15M) am the eldest between my siblings. My younger brother (13M), let's call him Matthew. And lately he's been emotionally closed off, often rude, and lately gotten worse.

He's being manipulated by a group of toxic friends who treat him like dog shit. They mock him, use him, and he keeps going back. I've tried to help him see it but he won't listen. And now? he thinks it's okay to take it out on me and our little brother (8M), let's call him rudie.

Rudie has additional needs and is really sensitive. He doesn't deserve this but Matthew's been snapping on him, like being cruel and acting like were the problem. It's like he's redirecting all the pain he gets from his "friends" onto us.

I'm tired. I'm trying to protect my siblings, and now I'm absorbing Matthew's anger too, I know he's hurting but it's not fair. I'm not his emotional punching bag and rudie definitely isn't either.

I don't know what to do anymore. I've talked to my mom, but it feels like im the only one seeing how bad its getting. I just needed to get this off my chest