What now? I am so sorry your birth mother is putting you through such emotional pain. Heartless.
Has she ever seemed a bit radical or non-conformist to you? Does she not get along with her "worldly relatives"?
If you get along with the worldly ones, I would show them the letter you showed us. That letter is shockingly horrible. That letter is evidence what a controlling, manipulative religion your mother has joined... Unless she is the one that is manipulative and is trying to control you to punish you and get you to come back.
Narcissists, if they feel they have lost face, got disrespected, will turn on that person who they feel is responsible. Did your mom feel like she lost status at the Kingdom Hall among the congregation(s), when you quit coming?
If your mom was always a kind and gentle soul and wrote her last Will ignoring you, then she is a 100% blinders on full in JW..org zombie.
Whether she is an angry narcissist or a full on WT zombie, what she did is hateful.
Your little children don't need to be around either one of those types even though it is their grandmother. Respect runs both ways. Bad behavior never should be rewarded.