@OP
Just my two cents but the letter doesn't come across as saying " Hey from now on, I'm going to shun you because that's the right thing to do". Instead it seems to say something like " The way you shun me is wrong, but I will make it easier for you going forward"
If you really want to take it up a notch, why don't you say that as a follower of Jehoover, you have come to realize that the Washtowell is not doing God's will, and so based on the bible, you now consider the Watchtower part of Babylon the great, and that since your mother is part of the world, you don't feel right associating with it. Also tell her that as soon as she returns to Jehoover by leaving her evil organization, you will welcome her back.