@maribellah,
If you really must insist on inserting yourself into this drama, do so by telling your brother that you know, and threaten to tell his wife if he doesn't within a short, set amount of time (say, by Monday).
If you need to carry out your threat, do so when talking to both of them; don't just talk to her by herself. But give your brother the opportunity to come clean on his own first.
I agree with tes yeux that this is not your concern, not really. I do recommend against getting involved. But, like I said, if it has to happen, do it the way I suggested. And be prepared for the fallout to come raining down on your own head. Unfair or not, that's often what happens, that the messenger gets shot.