What do we (as family members) do when we catch our soldier's wife cheating while he is away?

Well that is a conundrum.

Agree 90% with MSgt Hoffman. The other 10% is-- would the revelation really come as a surprise to your brother (or would he have seen it coming), and is it possible that they are in an 'open marriage' type arrangement.

I'm guessing the answers are yes and no based on the caution you take in posting the question, but just covering all bases.

DO NOT tell the wife you 'know' anything first. She's the one with Power of Attorney (most likely) so is in a position to trash your brother's finances if not life for the foreseeable future if she thinks she's about to be ratted out BEFORE your brother can take financial precautions against it.

Tell him so he can contact his financial institutions and limit her access to his assets BEFORE she suspects anything... If he indicates that he wants to immediately confront her when you tell him about it-- recommend he cut off her access to his finances before he calls her on it.

If you're friends with the wife I'd tell her to tell him ASAP. He deserves to know. I see the point others have made about waiting until after AIT but bad news does NOT get better with time. If she doesn't tell him, someone needs to. There will never be a "good time" to hear this. The sooner the better in my opinion.