I understand that it will be very difficult for you to do, but try to remember that you are only the messenger, your brother is the one hurting her and his family.
Start there. It'll give your brother an opportunity to end the A and come clean to his wife on his own. As a BH I feel there's a lot of value in having the WS end the A on their own and reveal it to us. It allows the lying to end sooner and gives everyone a better chance at R.
Good luck. There's no easy way out.
I don't know how your situation went down, but I *DO* know that most BSs confront too soon (myself included) with insufficient hard evidence and get gaslighted to hell and back.
Can you follow him and get pictures of the two of them together? You could do that and go help her pull up phone records, credit cards, refer her here, etc. and create somewhat of a timeline before she confronts?
At SOME point, she's GOING to ask if you knew, you know...
So, I understand you're between a rock and a hard place. Your brother will be TICKED, but just let it roll off and help his W- and help him- salvage their lives. At some point, he'll realize you did it to save his life, both literally and figuratively, and he'll thank you for it. It's going to take some time, though.
No longer together
Do not let others be your reference for who you see in the mirror.
Stop allowing people to hurt you, because you don't love you enough to walk away.