Son of an outlaw : a western story / Max Brand.

Enhanced descriptions from Syndetics:

Black Jack Hollis was a daring resourceful outlaw before he was shot down. Elizabeth Cornish, owner with her brother Vance of the Cornish Ranch, learns that the outlaw left behind a one-year-old son. She told her brother to find the boy and bring him to her.Calling herself his aunt and Vance his uncle, Elizabeth Cornish raised the boy she called Terry Colby.Vance Cornish was skeptical from the beginning, insisting that blood, not nurture, determines character - that an outlaw's son would inevitably become an outlaw himself. Building up the legend of the Colby name, making Terry believe that he was descended from the aristocratic Colby family of Virginia, Elizabeth believed that the life and world in which Terry grew up would form his character and make him all she wanted him to be.Elizabeth built the Cornish Ranch into a prosperous enterprise, while supporting Vance, who was interested only in travel and enjoying life. As a result Elisabeth had bought out most of Vance's share of the ranch. Vance doesn't care whose name the ranch is in because he believes that when Elizabeth dies, he will inherit his sister's wealth. But Elizabeth informs Vance it is her intention on Terry's twenty-fifth birthday to make Terry the sole heir to the Cornish Ranch. Vance decides he must make Terry aware of his outlaw heritage and, for Terry's birthday party, he invites a famous lawman as a guest ? the man whose career was launched when he shot and killed Black Jack Hollis.