Final answer:
Matt Ryan became Andrew's sworn enemy because he spread rumors that Anders had spent time in jail, as noted in Karen M. McManus's novel 'The Cousins'.
Step-by-step explanation:
In "The Cousins" by Karen M. McManus, Matt Ryan became Andrew's sworn enemy because he had D spread rumors that Anders had spent time in jail. This conflict stems from a series of complex relationships and actions that impact the characters' lives and the overarching narrative of the novel. The dynamics between characters are crucial to the plot's development, with enmity, secrets, and the power of rumors playing significant roles within the story.