AggieNostic: they don't even see how brainwashed they are.
Brainwashed people by definition don't know they're being controlled and manipulated.
That being said, all I can comment on that letter is this: Wow, what a manipulative letter!
She has certainly learned her controlling tactics well.
Likely any response will fall on deaf ears.
However, you might possibly point out that SHE is the once choosing to shun you, not the other way around. SHE is the once forcing the issue. Ask here where she gets off issuing ultimatums!
Ask her to show you a scripture where Jesus ever shunned anyone, even one person.
Her threat to not-invite you to their 50th Anniversary is just emotional blackmail. You might want to consider calling her on it.
Also--assuming you still believe in God--you can truthfully say, "We have NOT left Jehovah." When she points out that you don't attend meetings, ask her, "Is the WTBTS God?" After she says, "No," then you can point out that equating meeting attendance with belief in Jehovah is illogical and erroneous. Tell her you still believe in God. The cognitive dissonance will likely be overwhelming to her.
Change the subject. Talk about something superficial and pleasant. Ask her where the anniversary party will be ...
This religion is so messed up.
Hang in there!