Ocean of Lies by Ashley McCauley

A fake honeymoon, a missing step-sister, and an island paradise with danger around every corner.

For the past eight years, Sydney Davis has been living a lie. Just when she starts to enjoy her comfortable life in a gated city, her step-sister, Callie, is taken right in front of her. Desperate for answers, the recent college graduate turns to the mysterious man who witnessed the attack.
Luke Artino works for an off-the-record organization that specializes in preventing human trafficking in hopes that someday, he’ll forget about the one he couldn’t save. When his boss finds Callie on a resort island in the Caribbean, Luke volunteers to go. All he needs now is a partner.
With Sydney’s unrelenting determination and language skills that could be helpful on the mission, Luke reluctantly brings her along. The two pose as newlyweds at the private resort, but the constant surveillance forces them to grow closer than either was prepared for. As the mystery of Callie’s disappearance becomes more complicated, so do their feelings for each other.