The Crystal Keepers, An Overseers Novel

Loki is quite familiar with the forces of good and evil; she watched them clash among members of her family. Her brother joins the Guardians of Light while her sister joins the Brethren of Darkness, two orders that hunger for more power. Neither sibling is aware that Loki is at the center of a secret order whose sole mission is to keep the world in balance and prevent total destruction.