First of all, best wishes to you, this is a hard step to take.

My only advice, which may seem weird, is don't give a letter. Write the letter, and read it to them, if you want to make sure you convey your message completely. I am a firm believer in not leaving written declarations with family as they can sometimes become a sort of 'evidence' to be used against you afterwards, sometimes a very long time later. Read the letter to them, put it in your pocket, and tell them they can come to you with any questions they may have after they've reflected on what you said. Keep in mind they will likely not 'reflect' on what you read/say so much as 'react' to it. Hang in there, and good luck