The Fall Guy by James Lasdun

Although cousins, Matthew and Charlie were as close as brothers when they were teens. As adults they have drifted apart, but still have a bond. Charlie has done very well financially. He and his wife, Chloe, have a second home in the Catskills, and he invites Matthew to spend the summer. Matthew has not been that successful in his business endeavors and is struggling with what to do next. He also nurses an unrequited affection for Chloe. When Matthew discovers that Chloe is cheating on Charlie, he faces a dilemma: should he tell Charlie or keep quiet?  As the summer progresses, Matthew's knowledge almost becomes an obsession. There's not any way this can end well...