Viper's Daughter by Michelle Paver; the seventh book in the Wolf Brother series.
For two summers Torak and Renn have been living in the Forest with their faithful pack-brother, Wolf.
But their happiness is shattered when Renn realizes Torak is in danger - and she's the threat. When she mysteriously disappears, Torak and Wolf brave the Far North to find her. At the mercy of the Sea Mother and haunted by ravenous ice bears, their quest leads them to the Edge of the World.
There they must face an enemy more evil than any they've encountered.