free-
i wouldn't write the letter. i did write a letter to disassociate when i was 17 and now i have a sister and a brother who shun me. although for me at the time--since i was young--the letter served as a way for me to let my parents know that i was making my own decisions and they couldn't tell me what to do. Sometimes i wish i'd have just waited until i left for college so i could fade out of the org. but i have to live with what i did. you could write a letter in a journal or something and never give it to the elders but at least you might feel some closure by writing it down. the choice is yours, just think about your sister and what it would be like to not have her in your life(it sucks!).
good luck!
harmony