I don't know you, your problems, or your business but there are a few things that, as an outside observer, I can say are very clear:
You are being abused by your family. Yes some people have twats for relatives and everyone does what they have to do. But, seeing that you have serious health issues, your life can be more acutely impacted by this type of family garbage. Your mother is mentaly abusing you with what sounds like her game-playing and her apparent neglect in disciplining your brother. Your brother is physically abusing you, putting you at further risk than your health already has you.
I hear you, Kink, about the teenager needing support bit. You're absolutely right. He needs a lot of support. And he isn't getting it. If anything, he's in a highly detrimental environment that could make matters a lot worse. If mom's being a bitch and brother dear decides to break his rib playing tough guy, he's going to be in a much worse spot than he is right now. If his brother is hitting him hard enough to take his breath away, then he is a threat to his health, not support to it.
I say be selfish about your well-being. If no one's going to care for you, then YOU look out for you. If you're dealing with emotional and physical abuses, there are resources out there available to you. You have internet access. I'd say look into them. And take the power and the direction of your life into your own hands. Make sure your health and safety are your highest priority. Good luck with it. I hope things turn around for you soon.