Answer:
if i were in your situation i would just talk with my mom privately (you can ask your brother to join you as well), and tell them that you want them to separate because you guys deserve better. i feel like it would be a wrong idea to try to separate them yourself since they're the adults. maybe ask your dad and mom if they could possibly go to marriage counseling.
Step-by-step explanation:
also, i hope you're doing okay. don't let what your step dad says to you get to your head. life will get better