Hi Geevee,
Sorry you have to deal with this. My JW family knows that I have been irregular for a long time and they don't even care enough for me or my daughter to threaten me with destruction. So, at least this shows concern.
I don't know your situation, but if I got a letter like that, this would be my partial response:
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Dear B.,
I know you mean well and I appreciate your concern for my everlasting life. However, I do feel you are treading on dangerous ground. You see, Jehovah has authorized Jesus to do the judging, not humans. I detect a judgmental tone in your letter. It makes me feel that you're trying to shame me or guilt trip into returning. I trust Jesus' judgment of me and if that judgment is negative, and I don't end up being in the new system, then Jehovah will help you and the family cope with the loss. Each member of my family has the same decision to make and Jesus will judge them accordingly.
(I'd probably throw in a few scriptures on a Christian's responsibilities (not the WTS version of that) and on judging.)