Surfacing by Claire Cullen

A wolf trapped in the past. A merman fighting his true nature. They’re worlds apart, but they can’t survive alone…
Luke wants his life back, but he’ll settle for his freedom. His only chance of escape is Red, the fearsome, furious wolf searching for his stolen sister. He’ll say anything to convince Red to take him away from his human captors. And then slip away before his lies are found out. But Red has other plans.
Red’s search for his pregnant sister has hit a dead end. He knows who has her, but he needs leverage to get her back. And then he finds Luke. Red could never hand another wolf over to the humans, but Luke is no wolf, no matter how he makes Red feel. And with him, Red can strike a deal: the merman for his sister.