Sister Light, Sister Dark 1st ed.

Raised on a mountainside, Jenna learned the arts of the warrior, and from the mountain women the magic of the ancient lore. But the greatest magic of all was her ability to call forth her dark sister from the depths of the mirror of the land of light and shadow.

Skada was the dark one, able to exist only when the moonlight cast a shadow or lamplight flickered in a darkened room.