The sooner the better...it will only hurt him more if you wait. I'd try and get the cheating couple to fess up to him so that he doesn't have to find out about it from someone uninvolved.

As much as it will be eating you up, that he doesn't know? It's not your place to tell him of this. As much as you may dislike the g/friend too, you need to encourage her to tell him. Keep on it until something is done, but not by you - make sense? Otherwise you will be seen as the bad person...when you're not.

lol do what snookie n j-wow did...write it in an anonymous letter n then like 15 years from now when he's married n have a few kids tell him remember that time....lol

I would tell him right away, or when you want to, because there is never a good time to tell your brother..or anyone that there gf is cheating on him. Plus, if you were being cheated on wouldnt you want to know right away?