Destroying My Ex by Jordan Silver

When a teenage Grayson Carter was forced to continue dating the daughter of his father’s boss after she cheated, something in him snapped. For the next four years Grayson plotted and schemed for the day he would make them pay. He had no idea that fate was about to change his life plans and help him exact a more beautiful revenge than he’d anticipated.
Lily Devon Sinclair lost her father even before she was born to the mistress and daughter he had while her mother was pregnant with her. Now that daughter and her mother has made her life a living hell. When she sent Grayson the proof that her half sister was cheating, she had no idea that a year later they’d meet on the college campus and become more than friends.