I was cheated on by my ex, at least twice - with the collusion of some of our so-called mutual friends - and I wish someone had been honest with me. After the 1st time, I even spoke to one of these friends who actually apologised for not telling me, but said he didn't know what to do for the best.....
....But, but, but ...... in this case, unless your brother had actually confided in you, & despite his odd behaviour & history, you did not know 100% he was cheating.
Trouble is now, as he's gone back home, he's obviously trying to patch things up with his wife - and if he has told her that what you said was a pack of lies, it leaves her in a very difficult position. The way I see it, she has more or less 3 choices:
- to call him a liar and throw him out, ending their marriage.
- to believe what you said, but forgive him and try to make the marriage work ... which is always bloody hard after an affair
or,
- to believe her husband and for the marriage to continue as per usual.
Choosing option 3 probably seems, right now, to be the least emotionally painful choice for her, because she doesn't have to confront what she's heard - though from what you've said, there's a high chance her trust will be broken in the future. Of course, by believing her husband - that makes you, by default, a liar ..... and who wants a liar for a friend ? In other words, she could well be protecting herself by telling herself that you were out to cause trouble (no doubt this is what your brother will also have said) because it saves her facing up to the humiliation/hurt/broken trust/risk of STDs/betrayal that infidelity brings with it.
And of course, even if you had watertight proof he was cheating, your SIL could still go for option 3 and you would still be public enemy no.1. Some people also passively accept affairs, so long as they're not made public - and in those cases the messenger is bound to get shot.
It's a really really difficult call but I think I'd personally have taken the attitude of telling the truth (about stuff I was certain about)if asked, but otherwise kept counsel. And I would have also told my brother exactly what I thought of him too. I do know you had the best of intentions, but sometimes people just aren't ready to hear what you tell them ...... hence the waiting to be asked approach.
Maybe you could send her a card, just telling her you're thinking of her, hoping she's okay and apologising for anything you said which caused offence or upset, but which was said out of concern for her. Maybe also say something like you hope they both sort out their problems to the satisfaction of them both, and that you're still there for her if she wants to contact you.