The Otherworld by Abbie Emmons

Orca Monroe wants only one thing for her eighteenth birthday: to experience the Otherworld-the mysterious “mainland” across the sea that her father has forbidden her from visiting.

Growing up in a lighthouse on a remote island, Orca has lived isolated from the world… until one day when she finds a cell phone washed up on the beach. Orca has her first conversation with Jack Stevenson, a young man whose older brother, Adam, has gone missing after crashing his seaplane off the coast. Orca becomes Jack’s lifeline and his reason to hope that Adam is still alive. While her father is away, she scours the island for the missing pilot-determined to help Jack find his brother and prove to her father that she’s strong enough to take on the world.
One stormy night, Orca finds Adam Stevenson collapsed on her doorstep. As she nurses him back to health, she finds herself spellbound by his inquiring mind and rugged good looks. Simultaneously, Adam is captivated by her wild beauty and pure heart. But with a ten-year age gap between them-and her father’s determination to keep Orca protected from outsiders-Adam knows they can never be together.
Resigned to give Orca up, Adam returns to the mainland-but Jack refuses to leave her trapped at the lighthouse. Blind to the fact that his brother is in love with her, Jack offers to show Orca the world she’s always dreamed of. But when she leaves her island for the first time, Orca begins to realize that the mainland may hold more dark secrets than she ever imagined… and the two brothers she helped bring back together may be the very people she tears apart.