It's hard to see your MIL act out, especially if it's behind your husband's back. When this wife is conflicted, she asks Reddit:
My mom had a birthday recently and had a girls night. She wanted to invite my MIL, because they are becoming friends. I was really uncomfortable as MIL and I do not get along at all. It is bad enough my husband has had to go low contact, but my mom guilted me into inviting her.
We were at a fancy art studio with various lessons and wine and charcuterie boards, and MIL refused to wear her top (relevant later) so she had top that might have been a bra (I couldn't tell) and got resin all over her stomach. MIL freaked out and my mom went with her to try and help her wash it off (wtf)
I went into the bathroom as I wanted to wash my hands and overheard them talking
mom- your stomach is so flat. I hate you
MIL- I have sex with Aaron like five times a day. That is exercise.
mom- he's so pretty
MIL- kidding, kind of, but no he is very sporty so we do a lot of athletic date stuff, and I have to look good. He is so pretty. Aww it's almost six months
mom- marry him, do it
MIL- I actually feel like we might some day. I don't think I've ever been in love before this, and it's so sad at my age but oh my god I just love him so much.
At this point I wanted to throw up, because I know exactly who Aaron is and my husband is going to have a lot of feelings about MIL dating this man. My husband HATES Aaron, and Aaron previously laughed at him a lot growing up and made him feel insecure.
He used to flirt with one of my husband's girlfriends as a joke when he was in college (she was in love with him) He was always causing drama in my in laws marriage.
He made a joke at our wedding about my husband having no money. This asshole saw me stranded in rain once and splashed me with his car. My husband just hates Aaron, and I knew he was going to feel betrayed, but I also knew as my husband I had to tell him. AITA?
YTA you all sound like a bunch of drama queens. Just tell your husband, what's with the ultimatums and confrontations in the bathroom? This is like Real Housewives - 'we hate each other so we keep going to social events together so we can have public fights'.
I'm going to go with NTA, because I think once you knew, your options were limited.
If you had done nothing like some commenters suggest, then later posted 'AITA I knew my MIL was dating someone my husband hates but didn't tell him, now he has found out months later and is furious I kept this from him' you would be ripped to shreds.
Better be a called a 'nosy, immature woman who can't mind her own business' by the internet than called 'untrustworthy and dishonest' by your husband.
Yta. It was a private conversation you were purposely quiet to listen to. I can tell exactly why your mil doesn’t like you. And no she doesn’t have to tell her son who she’s sleeping with that’s only her business. Get yourself out of her sex life and get some manners.