These are the books by Catherine Fisher that are reviewed on this site:
A Viking mix of magic and superstition, courage and loyalty. A battle between Gudrun the sorceress, and Kari, the son who she disowned. Can Kari's powers possibly match those of his mother? And does he have to follow her evil path?
Gudrun has left the Jarlshold and returned to the land of the Snow-Walkers in the far north, beyond all natural knowledge. But she cannot leave Kari alone. She sends a rune-beast to fetch him, and doesn't care how many of the Jarl's people perish in the encounter.
The final conflict between the sorceress Gudrun and her son Kari. Can he develop his skills and match her power and still stay off the road to evil?
There's no love lost between Rob and his sister. But Rob finds, rather to his surprise, that he'll go to a lot of trouble to get her back. Magic and mystery.