Dear Mr. St.John-Black,
I have recently been informed that my sister has broken your engagement by some rather unconventional means. I do apologize for the inconvenience and the upset that I'm sure this development has brought you.

While I'm sure that her actions were in no effort to hurt you, I know that these words likely don't mean much. I do hope that you find a wonderful bride. She will be a very lucky woman.

With Care,
Miss Dorothy Finch