Depends on what you want to convey to her and on whether you want to stop hearing from her.
Your options would seem to be to keep hearing from her by following cyberjesus' advice (a friendly family letter that stresses the here and now and doesn't mention religion at all)...
... or to stop the communication by telling her what you've told us, that you are annoyed because she feels free to comment on your relationship with Jehovah any time she sees fit but won't even listen if you try to point out inconsistencies from the Watchtower's own publications.
If you do want to cut off communications you might also ask her if she wrote that letter on her own initiative or on organizational instructions. For example, how much time is she reporting for having written it?