He's a cheater! Should I tell his wife?

Would you tell your sister in law that her scoundrel of a husband had an entire emotional affair a few months ago that your (currently separated) husband/his brother knew about !(this part isn't as relevant, but I'm still flabbergasted). She's about 6 months pregnant. This whole thing happened Feb/Mar which is more disgusting to me as not too long after she got pregnant! I actually found out in a crazy way...met the girl through some political networking, and she told me everything last week. Men are really out here dumb as rocks I tell you.🤦🏽‍♀️I've really been struggling with whether or not I should say anything, they have 2 kids already. I really don't want to stress her anymore than she is however, if it was me I would pray she would tell me. I actually want to kill him, I think that would save us both some trouble 😩🤷🏽‍♀️

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