Not much of an answer but who's been worrying about your brother for the last 20 years?
It would appear that your parents have dropped the ball. If anything happens to your brother they'll be blaming anyone but themselves.
Your responsibility is to your current family. If the Police need to be involved, so be it. Unless you are responsible for introducing him to his dependency you can't take the problem on as yours.
If he shows up at a Soccer game, and is "under the influence" your best move would be to use your cell phone, not your gun. Call the Police and let them deal with the issue.