Sister Light, Sister Dark (Great Alta, #1)

Then Great Alta plaited the left side of her hair, the golden side, and let in fall into the sinkhole of the night. And there she drew up the queen of shadows and set her upon the earth. Next she plaited the right side of her hair, the dark side, and with it caught the queen of light. And she set her next to the black queen.
"And you two shall be sisters. You shall be as images in a glass, the one reflecting the other..."

Raised on a mountainside, Jenna learned the arts of the warrior, and from the mountain women the magic of the ancient lore. The greatest magic was calling her dark sister from the depths of the mirror of the land of light and shadow. Skada, the dark one, existed only when the moonlight cast a shadow or lamplight flickered in a darkened room.