Should you tell your brother in law that your sister in law is cheating on him?

Anonymous
So obviously this is weighing really heavily on me. I honestly feel horrible because I knew she was cheating on him before they got married, and he got her pregnant. She told her brother that when they got married she would stop, and so far since then I thought she had changed. Well my husband, and his sister got into a really bad fight recently, and afterwards he let it slip to me that she is in fact still cheating on her husband. Then proceeds to tell me how she tells him she’s so happy she can live a double life, and her husband will never find out since she goes out of state for work, and that’s where she meets the guys she cheats on him with. He then starts telling me details about how she’s so proud about how she just started seeing this new guy who is married too, and has a baby, and how no one will ever find out the truth. This whole situation is so sad to me, and with the holidays coming up it’s just going to make things incredibly painful for me to be around them knowing what she is doing to her family. I honestly wish he never said anything to me about it, but now that he has I don’t know how to live with myself knowing this, and not doing anything about it anymore.
Updates
+1 y
Found out she wasn’t just cheating with random guys but she’s been sleeping with my husband too her own brother. Wow crazy what you can find out when you speak up I got beat for it by my husband but at least he was mad enough to finally tell me the truth about them because he wanted to hurt me for coming after his sister “cough” lover. I feel so bad for our son because I’m going to have to tell him when he is older that THIS is the reason his parents aren’t together.
Should you tell your brother in law that your sister in law is cheating on him?
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