Fantasy world where teenage boy is taken from family

The book is a fantasy about a boy who lives on a farm with his family. One day ‘priests’ come to test the children to see if the have ‘power’ and the boy is taken from his family as he is shown to have strong powers. The priests take the boy and after leaving the father packs up the family so that they can follow them. The boy learns that there is a girl being held in one of the wagons. That night when they camp the boy is confronted by the ghosts of his family. When they left to follow him they were attacked and all were slaughtered. He is beyond devastated but has to help drive back the attacking ghosts.
We later learn that the girl in the wagon was out looking for her sister, who can turn into a dragon. She has been kidnapped. She herself can turn into a dragon but cannot control herself if she does
The ‘priests’ bring them back to their stronghold and begin training the boy. He is basically turned into a bounty hunter and when his training is finished he is given names of criminals he has to hunt down. We learn that one of these criminals is his father who survived the attack on the family

Can someone please help me