That's very bold of you, even through a letter.
I remember when I came out to my sister, I did it gradually, over a period of days, though an e-mail exchange.
I'm currently in the process of coming out to my monther, even more slowly, through telephone conversations and Skype videoconferences, week after week. She was very surprised at seeing my budding breasts at first, and she is still worrying about my health... I still have to expose her to the plain truth of my GID problem. I hope I will succeed before my FFS.
As for friends, I already came out to a few of them, lost part of them...
But I made new friends, in the TG community.
Transition is really a change of life.