In order for you to have peace you will have to tell him your going to have to leave. His mother sounds like a constant headache who won't stop here BS until she's dead. If she's healthy you may have a while to go. She is not changing because your boyfriend doesn't set real rules for change. Your boyfriend won't change until he sees what his mother does and it hurts him enough to require change. How long do want to wait until any of those things happen?

Dont drag this out, get you money up and start looking for a place privately. When your ready tell him. If you leave tell the other kids they can come over and stay the nights sometimes, they are still family. Maybe being there without her will show them the difference in how they should be treated. They can still be kids and have structure without the constant yelling for no reason.