But maybe karma exists

Having been part of a local music scene, I think OP’s in serious denial about what’s happening. It’s likely the newly single bro is specifically hitting up OP because he knows about the past affair and figures she’s easy. If he really had a crush on a groupie for years, he likely didn’t get involved since sue was his bandmate/bro’s side chick for 5-6 years and the wife found emails. This guy likely isn’t expecting for this to be some LTR where he’s worried about the women meeting, and if he genuinely doesn’t care, it makes me think that he’s got some beef with his brother and is trying to stir the pot.