The Oracle Prophecies by Catherine Fisher

Mirany has been chosen to be the be the Bearer of the God, one of the Nine, a group of 9 priestesses held in the highest regard for the important roles they play in allowing the God's wishes to be heard. When the Archon, the living embodiment of the God, dies, the God chooses a new young boy to carry him. Mirany soon discovers a plot to place an ordinary young boy as the Archon, someone easy to manipulate, someone who isn't the God at all.

Seth, a scribe who's made a dangerous bargain in an attempt to help his ailing sister, winds up getting involved in Mirany's attempt to carry out the God's wishes. Not a figure of known significance, like Mirany, he is better able to move about undetected.

In this time of severe drought, Mirany and Seth might be the only people who can defy the corrupt plans of those in power and restore balance to the land.