Nietzsche, the philosopher, might be able to help. By what you said, that he’s highly creative but at the same time seems codependent, his writings may put into words what he might be experiencing. I’d suggest Genealogy of Morals as a start. He might finally feel like he’s been ‘heard’.

This is a very offshot reply, and I don’t know if it’ll work, but I think that it’s worth a shot.

If you’ve done everything and there’s been no avail, I think that you’ll need to go up to your brother and tell him about how you’re feeling and tell him what you wish would happen. If this doesn’t, then I think that you may need to let him go. I know you suggested that your parents are also intertwined with this in a weird way so I’m not sure, but that’s the best that I can think of.

In the end, I feel that you may need to let him go and let him figure it out, but your judgement is better than mine.