I'm so sorry this has happened to you, I think you are right the recent conventions have caused complete misery for many in your situation - WT made It abundantly clear that active witnesses are the only people you should associate with, family or not.
They are tightening the noose.
I would word a calm, loving letter stating that you no longer identify as a Witness
. You could also mention that research into the past and present practices of the org has made it impossible that you ever will. Shunning the inactive is the very opposite of what Christ commanded. Forgiveness and self sacrificing love. Not a self serving obsession with reserving your first class ticket to paradise and the hell with those you love.
All you can do is ensure she knows your door is always open and that you do not recognize the commands of men. Interestingly I found this today, small article about the man whose pyramidology Russell used/stole. May be worth sending to sis...her beliefs are based on utter pagan superstition.
https://en.m.wikipedia.org/wiki/Charles_Piazzi_Smyth