Yup this, my MIL is a pretty shitty person, cheated on my wife's father right around the time he was dying of cancer, when he did die she kicked my wife out of the house (she was 19 at the time and just lost her father) sold it and moved 4hrs away, didn't tell my wife where she had gone, no contact number, then proceeded to "poison the well" with her side of the family.
We recently found a letter that was supposed to go to me but never made it, from my MIL's sister saying how bad a child my wife was and how she made her mother's life so difficult, all bullshit, plus they didn't talk to my wife for years after the cheating started.
Thing is, my MIL runs a facade, it wears off in a few days if your staying with her and her narcissistic tendancies come out. She is still with the man she had an affair with, and I refuse to call him family, hes my MIL's partner if I ever need to explain it, which is often atm because hes almost on his death bed from all the alcoholism and smoking through the years
Here's the thing. If you allow the cheating partner to leave without explaining it, they're gonna use that time to try and poison the well. They'll start spinning stories about why they left -- stories that paint the non-cheater as an abuser, for example. They have to bring the non-cheater down so that when the kids DO inevitably find out, they're left believing that the split was mutual.
By forcing her hand immediately, you made sure she wouldn't have time to create "plausible doubt" in the kids' heads. That's why she's really upset, not because they found out (they would have inevitably found out), but because she didn't have a chance to make you look as bad as she was.
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