You need to truly accept the fact that it is none of your business.

Your brother's failures are not your business and not your problem. You parents' decision to coddle to him is not your problem and not your business. Your grandmother's choice t house him is not your problem and none of your business.

It is very important that you manage your relationship with your family in a realistic and proactive way. Your mother will want to talk about your brother. Don't engage. Your parents aren't that interest in you. That's OK. Be civil, keep visits brief, and possibly reduce the frequency.