Mountainfell by Katharine Orton

There is nothing more dangerous than magic... Eleven-year-old Erskin is used to danger; she lives in the shadow of Mountainfell, a place of wild creatures and dangerous magic. When the most powerful and deadly creature of all - the fearsome cloud dragon - snatches Erskin's sister away, Erskin must face her greatest fear and journey onto the mountain to bring her back.A terrible power is stirring, and it is up to Erskin to save both her family and her home