Heirs of Mana Series by Matt Larkin

She controls the seas. Her sister controls the flames. Together, they rule as god-queens over their island nation. No mortal army can stand against their power. But what happens when civil war erupts between these goddesses? Namaka turns the fury of the sea on her sister, wreaking untold devastation on the land and under the sea, earning the ire of the mer kingdoms. Their answer: turn Namaka into one of them. Possessed by a mermaid spirit, she is drawn into battles in their alien world... moreShe controls the seas. Her sister controls the flames. Together, they rule as god-queens over their island nation. No mortal army can stand against their power. But what happens when civil war erupts between these goddesses? Namaka turns the fury of the sea on her sister, wreaking untold devastation on the land and under the sea, earning the ire of the mer kingdoms. Their answer: turn Namaka into one of them. Possessed by a mermaid spirit, she is drawn into battles in their alien world. How will she survive a war not only with her sister, but with a rival mer nation? You’ll love this gritty dark fantasy that unfolds across tropical islands and undersea realms laced with hidden savagery. less

Her rage is an inferno. Flames and volcanoes answer her call. Armies fall before her. But not even can she can raise the dead. Can she? With unlikeliest of allies, Pele strives to save the soul of her fallen sister. Time is short and with every passing hour, the girl drifts closer to becoming a shade. All that might save her is the Waters of Life... moreHer rage is an inferno. Flames and volcanoes answer her call. Armies fall before her. But not even can she can raise the dead. Can she? With unlikeliest of allies, Pele strives to save the soul of her fallen sister. Time is short and with every passing hour, the girl drifts closer to becoming a shade. All that might save her is the Waters of Life. But how far will the immortal guardians go to protect their secrets? And even if Pele can overcome the gods, can she face the mistakes of her own past? You’ll love this gritty dark fantasy that unfolds across tropical islands and undersea realms laced with hidden savagery. less