This is book number 3 in the Faelin Chronicles series.
Mage Fire is the thrilling conclusion to The Faelin Chronicles series about a twin boy and girl, the children of a Fae mother and a human father, who discover a new destiny after their parents are murdered.
They learn that their parents' cautious ways hid a lifetime of secrets, including Eirian, a magical sword that was buried on their farm. Their little sister has been taken by someone who wants that sword, and the ultimate power that comes with it. The twins--along with their friends, a goblin boy and talking wolf pup--must recover the sword and use it to save their sister.