Valentine by Kimberly Loth

Three kings will arise. One from the land. One from the sea. One from fire.
Hazel’s entire world is turned upside down after her brother Rowan goes missing. Everyone believes he was eaten by a dragon. Everyone but Hazel’s sister, Aspen, that is.
Hazel has never heard of the dragon prophecy before. She knew about dragons, of course. Her sister, Aspen, made sure of that even though Hazel would have preferred to stay ignorant of the vicious creatures. But the more Hazel learns about Aspen and their family’s connection to the dragons, the less she wishes she knew.
Val never wanted to be king. He never even considered it a possibility. But now the prophecy’s fulfillment has begun, and it starts with him. His journey begins by training with Obsidian. In the process, he learns secrets from his past that shatters everything he knows..
On the way to mainland from Hawaii, Hazel and Val’s worlds collide and will soon have to rely on each other to find Rowan and prevent a human-dragon war.