I tell most people to mentally prepare themselves for loss of relationships with family members. In a lot of cases this is a good thing, as your relationships were originally built on a false presentation of yourself.
Points to include in letters may be;
1) That you are not asking them for input or assistance making the decision, and that you are firm on your decision to proceed (if you are of course.) --This lets them know it is not a debate, nor a decision they are included in making.--
2) That you desire to have them continue to be a part of your life should they be able to treat you with dignity and respect for the woman you are (important to refer to yourself as female.) --This lets them know that you both value them, but that you have a certain standard of how YOU want to set the tone for your interactions with them.
3) Maybe something that can put them at ease, like offering to be easygoing if they slip with your name or pronoun, but also tell them the correct one. --This lets them know that if they can give you the dignity mentioned above, that you understand that the process of changing their entire memory and existence knowing you is a process, and it takes time, and in that time, you will be respectful of them and how your change complicates things for them.
A lot of people shoot for something heartwarming, the perfect thing to say, that sounds good when you read it. I think you are better off laying guidelines, and putting fears to rest in your correspondence that trying to make it poetic.
Prepare for the worst, hope for the best and good luck!!
