Harris Joanne. Runelight

Three years after the events in Runemarks, the Middle World has changed again. After the destruction of the Order, the surviving gods, having rescued their friends from the Black Fortress of Netherworld, have regrouped in the village of Malbry. The General is dead and chances of rebuilding Asgard seem remote. But a prophecy, coupled with the appearance of unnatural creatures in the Middle Worlds, reveal a breach in Chaos, which may lead to another Apocalypse. Once more, Maddy and her friends are forced to confront an enemy. This time, the enemy is one of their own, a renegade from World's End - Maddy's twin sister. They are both the daughters of Thor."Six hundred miles apart, two girls each bear on their skin a runemark: a symbol of the Old Days when the known Worlds were ruled by the gods from their sky citadel, Asgard.
Now Asgard lies in ruins, and the power of the gods has long since been destroyed.
Or so everyone thinks.
But nothing is lost for ever, and the gods haven't given up yet (nor stopped squabbling!) and they want the power of the runes borne by Maddy and Maggie - these new runes, which carry huge potential, their runelight shining out as a portent to the future.
Soon both girls are swept into a maelstrom of cataclysmic events that are to draw them closer and closer to each other, and nearer and nearer to a horrific struggle where each must prove where their loyalty lies...
Filled with inventive and humorous detail, trickery and treachery, carnage and lunacy, Runelight is the second title - following Runemarks - in a series of gloriously imaginative and dramatic tales about the Norse gods."