You need to get some one on one counseling first. You have been terribly victimized. Sending an "accusation of rape letter" to this person's family is not necessarilly illegal assuming the facts are provable but such accusations will put you in jeapordy of criminal and civil inquiry i.e. open a can of worms against you. Don't think you want that.
In my opinion, you should contact a good civil litigation attorney in New York since you do not want the plubicity and burdon of proving rape in a criminal trial. You can sue this Dude and the burdon of proof is reduced to a preponderance of evidence. Talk to an attorney before you attempt to satisfy your anger via a hate letter to the dude's relatives.