You should ask your bro about it first. It'll be easier as bros to talk man-to-man other than with your parents, where they can both team up on you if shit goes down. It's not always easy to be straightforward with parents, I understand.

If your bro isn't the kind of person to make solid oaths of silence, or you can't trust him, you're best off telling your parents straight up or finding some dirt on your bro that will make him think twice about ratting you out. Best solution is the former, though.