Oh, I have proof. I have several emails that give a nice timeline of the beginning of the affair (while it was still EA), which I got before he changed all his passwords and stopped bringing home his laptop (and then moved out). I have those printed out, in case he ever gets into my emails.
I have the email I received from OW in answer to one I sent her; doesn't explicitly admit an affair, but it is quite implicit in what she writes.
I have D's report of H freaking out when she was looking on his cell/blackberry for a digital game, then yelling at her to "never ever look at my email." Okay, that isn't explicit either, but kinda supporting evidence.
And I have my bank statement, which lists the times he has stopped for wine and take-and-bake pizza near her home--which is far on the other side of the furthest west suburb of Indy; we live on the east side, don't know anyone else out that way.
So nothing explicit except for emails, but those give a clear picture of how it began.
I sent those emails to H's brother a few weeks back, for the very reasons you suggest. Initially the brother was shocked and supportive, but after awhile H talked to him, and he and his wife have cut me off. His parents don't know yet; they will have a cow. I have thought that probably he should tell them, but he'll probably tell them whatever he's told everyone else. Either way, they're Italian; blood is thicker than water, he's the baby boy and can do no wrong. They won't be happy--no divorces in his family, going back several generations--but they definitely won't change his mind. And the common enemy will be me. With my bil and sil, the damage has been done, and even if things work out for the best, my relationship with them is fractured.
I also sent the emails to his pastor--for the same reason. When I talked with him, he was quite supportive and very disturbed about the MLC. I don't know what happened when H and pastor talked, but he did express things were okay "once he was able to explain himself."
I do wish I could do a tiny bit of info gathering, for this purpose, and find a very explicit love note; given that it's undoubtedly a PA now, I would imagine those notes are being passed. But I don't have any way to do that. And it's probably just as well; it would be hard for me to read.
I predict.....that will be short-lived in the grand scheme.
sg Love is PATIENT, love is KIND, LOVE never fails / DB since 2001