Thicker than Water by W.C. Tuttle

Angel McCoy rode back to Red Arrow, his mind filled with mixed emotions. Although it hurt him deeply, he was obliged to admit to himself that his father had out-gamed him. He tried to explain to his conscience that the whole thing had been a colossal piece of melodrama, and that he feared to get the ace of spades. He was a good shot. There was little doubt in his mind that his first shot would settle the whole argument, and he would be branded as a murderer.

There had never been any love lost between himself and his father. Their natures had always clashed. But Angel, even with his cold-blooded nature, did not want to be branded a parricide. The whole thing seemed so ridiculous now. Lila had been away to school for five years, and had returned a beautiful young lady, fit to turn the head of any man in the country. She was not his sister, and he could conceive of no reason why he should not marry her -- if she was willing. She knew now that Rance McCoy was not her father, and, being of age, could do as she pleased.