In the Roar of the Sea by Sabine Baring-Gould

The strange, curt note from Cruel Coppinger served in a measure to divert the current of Judith's thoughts from her trouble about Jamie. It was, perhaps, as well, or she would have fretted over that throughout the night, not only because of Jamie, but because she felt that her father had left his solemn injunction on her to protect and guide her twin-brother, and she knew that whatsoever harm, physical or moral, came to him, argued a lack of attention to her duty. Her father had not been dead many days, and already Jamie had been led from the path she had undertaken to keep him in.

But when she began to worry herself about Jamie, the bold characters, "C. C.," with which the letter was signed, rose before her, and glowed in the dark as characters of fire.