Most Helpful Guy
I'd let the brother bring it up, if he brings it up at all. There's a chance he may not bring it up. Even if he does, your friend may not bring it up with you. I don't see the importance of raising the subject personally.
Most Helpful Girl
Well, you can contact his brother if you still talk, and tell him you're seeing his brother and the two of you can decide what to do. however it wouldn't be smart to hide it, if you guys end up dating long term x