Okay, I know you're not gonna want to hear this. It's gotta be said.
You need to kick your brother's ass. And I don't mean punch him once or twice. I mean, quite possibly, put him in traction. From what I can tell, he's in a state where he thinks he's king. He's not. Someone needs to make a man out of him. He's not going to listen to reason. If you challange him, he's going to run. You need to chase him. Beat the holy **** out of him. Since you're over 18, yes, this could very well get you in a lot of legal trouble. But if your folks are not for using meds, and your brother doesn't know his ass from a hole in the ground apparently, someone's gotta stand up to him. With no disrespect meant, it's pretty obvious your dad isn't going to, and it's not your mom's place. Once you show him that you [as a group] are not scared of him, then he's ess tee eff you. From there, you can take more rational paths to saving your family. Blood may be thicker than water, but it's no fun to swim in, if you get my meaning.