Judging from the fact your mom & dad have abdicated their role as parents (and pushed some of those responsibilities on to you over the years), it sounds like your anger at your little brother is misdirected. You're really angry at your parents. Not saying your brother isn't being a little shit, but he's an 11yo with ADHD & virtually no parental guidance, so I think your expectations for his behaviour are unrealistic. It's not his fault your parents dropped the ball with him (after expecting perfection from you & your sister).

If it's to the point you feel hatred around him, the simplest thing to do is limit contact. If & when your parents actually notice you're not coming around, consider having a come to Jesus talk with both of them. Ask them what changed so much that little brother gets the 'free range' treatment while you & sis had to be angels. Ask them if they think it was fair that you had to spend your childhood literally cleaning up his shit when he's not your son, he's theirs. Let them know by ignoring their son now, they are setting him up for failure (as well as setting themselves up as his caretakers long into his adult life). He is their responsibility, not yours.

I think this is the real source of your frustration, not your brother.