I said this to someone else, recently.
Avoid the strange JW impule of letter writing.
Yes, you want to lay out an argument, a detailed, nuanced argument explaining yourself.
DONT.
buy a journal.
then continue to have face to face conversations with your doubting family.
Letter writing and giving does nothing but produce evidence against you. The cult mentality can snap back on again at any moment, and the person may feel the need to report you or burn it or you name it to your letter. In the present moment, when things come up, you can draw them out and try to share these taboo ideas. There are small windows where a believer has doubts. Most people cannot live in a state of uncertainty and close their minds back up to something more comfortable- in this case the lies and tragic message of the WTS.
I believe letter writing must come out of the JW mindset that you are the speaker and your audience needs to listen, and you will line up your points and scriptures like giving a talk.
This is really not effective communication, iit is one sided. Trying engaging in a conversation, where there is an exchange and you listen and respond to the person and what they need from you rather than presenting your case, as you say.