The WT Org. is a very dangerous cult that is killing in the name of God. Of course, they don't need physical weapons to do that -- their disfellowshipping policies alone are often as effective as the electric chair.
For this reason, I believe I have a social responsibility to warn as many as possible about what goes on inside the Organization. Complacency is simply not an option when so many lives, families, are at stake. Like many others who contribute to this site, they put me through hell, and I don't want one person more to have to suffer as I did. Thankfully, I got out of that hostage situation without the loss of any family members (I was the only JW), but many do not, and this has often led to tragic consequences. I realize I am not going to change the world by warning others, but if I can help even one person avoid potential death by disfellowshipping, I will be accomplishing something noble and worthwhile.
With that in mind, should you send your letter? By all means, as far as I'm concerned! However, I do agree with some of the others that a different strategy is needed because a defensive response is likely if you send it as is. Do you know of anyone that can help you revise it so that a light bulb goes on (hopefully) rather than her thinking faculties switching off? Either way, PLEASE don't rush it. Carefully think about the things that matter most to your cousin then use this information to develop an approach that will have the greatest chance of success in reaching her.
You are to be commended for caring enough to do something about what you know is terribly wrong. In the end, whether you reach your cousin or not, at least you did your part to sound the warning about the many social injustices that are being committed by the WT Org. on a daily basis.