I sent my Ns Susan Forward's "cease and desist" letter.
The "Cease and desist" NC letter (as found in her book Mothers who can't love chapt 13 - The most difficult decision), should only state coldly, that you've decided for your own good to break all contact and that from now on they shouldn't try to contact you nor call you for family events, and that from now on you won't see each other. The template for that letter can be found in her book. My copy is in French, but I can try to retranslate the letter template to english for you if you wish (the text above is the gist of it, not the proper wording).
And I sent NC letters to both Nsis and Nmum, and they were effective. I think what helped was that they do not point an accusing finger towards the N, they just state that you are going NC for your sake, and what NC means. (as in France the legal "Cease and Desist" action does not exist, I sent it through a postal mail service that tells you when they get the letter, but that they do not have to sign up for, so they cannot "refuse" your NC - Susan Forward insists it works better when sent in paper, the physical object of the letter helps getting the message through to the N apparently)
They have only tried to contact me a couple of times since then, for funerals and similar.
https://smile.amazon.com/dp/B00BATILFG/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1
I know what it's like, OP. Please read it.
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"they're too young, I never would have let you see something like that at their ages".
response - "Good thing you aren't the parent here. I am making the decision."
"no, don't put her in daycare. Poor baby. I would have never put you in daycare".
response - that was your choice. I didn't ask for your input on mine.
"wow, must be nice. I had to stay up late every night to clean".
response - it IS nice, why thank you mom. It's so nice to hear positivity from you.
screams, yells, doesn't talk. I just thought we were having a conversation mom but if that's not something you like, we don't have to communicate.
See, the thing is, she's not going to cut you or your children off. She has nothing without you and them, you being a captive audience who has been trained to put up with her toxic spillage and misery - she can't let you live without putting you down about your choices because how else would she feel good? and them, love sponge machines who are easy to feel superior over and are, anyways, still in the cute loving phase before they get real opinions which is when narcs will turn on them.
she gets mad and doesn't talk to you? WIN. Tell her, "oh, we're talking again are we? It was such a nice holiday, don't you think?"