I hate not being positive but this line sticks with me, "They're selfish people who only care about themselves"

The fact that they are taking in and caring for a physically, emotionally, and mentally challenged son AND helping you financially doesn't support that. Where would you have your brother go then?

It is NOT a parent's responsibility to care for or financially support an adult child.

I am not dismissing that there is trouble in the relationships, especially your feeling your own relationship with mom and dad has been compromised by your brother's difficulties. Is there anything you can do to participate in the situation that might build the bridge from your end?