Frank Merriwell's Brother by Burt L. Standish

That message gave Merriwell the greatest surprise of his life, for it told how Charles Conrad Merriwell, Frank's father, after his first wife's death, had married another woman, whom he met in the West. And it told how, by his second wife, Mr. Merriwell had had one son, Richard, who was thus a half-brother to Frank. But Mr. Merriwell had been hunted by his bitter enemy, Dion Santenel, and never had his life been anything but one of trouble and fear. It was not such a life as would make a wife happy and contented. Fearing Santenel might find his wife and strike him, through her, Mr. Merriwell had hidden her away in a safe retreat. But she was frail and delicate, and she had not survived.

The second Mrs. Merriwell was a sister to the wife of the man known as Juan Delores, and so to Juan Charles Merriwell took the motherless boy, Richard. Juan had raised Richard there in that hidden valley as if the boy were his own son, and there he had been happy and contented, with Felicia, his cousin, for his only playmate.