The Brave by Nicholas Evans: you may recognize the author's name. He wrote The Horse Whisperer. This novel isn't about horses but rather about a cowboy loving kid.
Tommy loved his sister Diane almost as much as his cowboy TV shows, his escape from his difficult life. When she showed up at his boarding school with her new boyfriend, Flint McCullough, he was amazed. That was his cowboy hero! And when they moved him to California to live with them, he was living his dream.
Until he found out about the family secret.
This is a complex novel with intriguing characters, interesting background and setting, and a deep theme. The viewpoint changes from Tommy as a teen, a younger child and as an adult. So the astounding ending is hinted at but not revealed until the bitter end.