Sister and her Ex.

If the guy is really that emotionally attatched to her. He would let himself believe there is a chance to get back together, and whatever other people do or say won't matter to him. So, even if you send that letter, I would assume that he would just tell you, it is non of your business and continue bother your sister. The only thing that can help is time, this is coming from personal experience.

As time goes by, the guy should cool off and forget about your sister. For me it took about a year, and my case is more like phycial attatach rather than emotional.

How long has it been off for? If its more then a few months, then the guy needs help. If it was recent, just let time take its effect. That or hire a stripper for him to move onto.

I feel madly in love with the stripper that gave me my first lapdance.

For about a week.