Thirty Sunsets by Christine Hurley Deriso

To Forrest Shepherd, getting away to the family's beach house with her parents and her brother, Brian, is the best part of every summer. Until this year, when her mother invites Brian's obnoxious girlfriend, Olivia, to join them. Suddenly, Forrest's relaxing vacation becomes a mission to verify the reality of Olivia's rumored eating disorder. But the truth behind Olivia's finicky eating isn't at all what Forrest expected. And over the next thirty days, Forrest's world is turned upside down as her family's darkest secrets begin to come to light.