Disrespectful brother

I'm 23 and married. My brother and sister's dad does not pay child support, and barely has anything to do with them. Since I love my siblings so much and practically raised them from the day that they were born I have chosen to help my mom out alot with them. It's like they're my kids is guess because I'm always going out of my way to please them. Pick them up from school most days, help them with homework, buy them food, clothes, supplies, etc. My husband is so generous he even lets them go on vacations with the two of us. They are 12. And here recently my brother has been letting his smart mouth get the best of him. He knows every thing and always has to have the last word. He's lazy and his life revolves around playing games on his phone or watching tv. When I suggest going outside to play he just pops off at me. I won't even be doing anything to him, I'll be cooking or cleaning when they stay with me and he will make remarks at me about my appearance or call me stupid or the things I do stupid. I don't know if it's just the teenage years or that he's grown up hearing our mother talk to us like that. (To be honest, she was a terrible mother, but she's trying now.) I don't know how much of this I can take before I tell my mom I can't do it anymore and to find someone else to help with him. I'm afraid if he keeps treating me like crap my anger is going to get the best of me.

***My mom doesn't want me to discipline him. So when I tell her he was bad today she just says, "Oh, I don't want to hear it!"