Boyfriend War — Francine Pascal

Jessica Wakefield and her best friend, Lila Fowler, are at war! The battlefield is the white sandy beach of Club paradise, a fabulous island resort owned by Lila's uncle. Jessica's dreams of lounging by the pool and dancing the night away are shattered when she learns that Lila's uncle has less relaxing plans for them: Jessica and Lila aren't guests, they're hired hands! Jessica's going to make Lila pay for this, and she's determined that the price will be Lila's new Club Paradise boyfriend.
Back home in Sweet Valley, Jessica's twin sister, Elizabeth, makes a shocking discovery. While researching her family history, she unearths a secret about her mother's past that could change the future of the Wakefield family.