Siren Trilogy by Tricia Rayburn

Vacationing in Winter Harbor, Maine, is a tradition for Vanessa and Justine Sands, and that means spending time with the Carmichael boys. This summer, Vanessa is determined to channel some of her older sister’s boldness, get over her fear of the ocean, and maybe turn her friendship with Simon Carmichael into something much more.
But when Justine goes cliff-diving after a big family argument, and her body washes ashore the next day, Vanessa is sure that it was more than an accident. She is more certain of this, when she discovers that her sister was keeping some big secrets and Caleb Carmichael’s gone missing. Suddenly, the entire oceanfront town is abuzz when a series of grim, water-related accidents occur, with the male victims washed ashore grinning from ear to ear.
Vanessa and Simon team up to figure out if these creepy deaths have anything to do with Justine and Caleb. But will what Vanessa discovers mean the end of her summer romance, or even life as she knows it?

After losing her sister and finding out who- or what- she really is, Vanessa returns home for her senior year. Everyone at Hawthorne Prep has questions about her tragic summer, but knowing who to open up to is impossible. As she walks the halls without Justine for the very first time, Vanessa has more to worry about than just making a new life for herself.

Because the sirens are out for revenge. To protect herself and the ones she loves, Vanessa must call on powers she believes she has but does not know how to use. To learn, she'll have to seek help from the only person who truly knows her. But first, she must admit the truth about her real identity.