Soul Survivor by Susan Faw

When sibling rivalry spills over into the land of mortals, the mortals lose. Great evil, spawn of the underworld, is poised to sweep all life from the planet, and only the twins can stop it. Jealousy, love, hatred and magic battle for the ultimate control of the souls of the world. Only one will be left standing, and if it is the wrong one, how will the world survive?