Oathbreakers by Mercedes Lackey

From the publisher (source: The Internet Book List):

SUNHAWKS' QUEST-
When Idra, learder of the crack mercenaries known as the Sunhawks, failed to return from a journey to her home kingdom of Rethwellan, Tarma and Kethry, warrior and mage, set out in search of their vanished leader. Sister of sword and spell, their fates bound together by a Goddess sworn oath, they were eternally pledged to fight the forces of evil.

And evil had indeed cast its shadows over Rethwellan. Idra, so they were told, had left long ago on a search for a legendary magical sword which could reveal which of her two brother was meant to become the new king. With the princess gone, her older brother had been branded an outlaw and her younger brother had claimed the throne. Both instinct and mage-lore told Kethry and Tarma that all was not as it seemed, that both Idra and her people were in terrible jeopardy. Yet would their Goddess-given powers, aided by those of a Herald of Valdemar, prove strong enough to break the dark enchantment possessing this land?

This book is part of the "Valdemar: Vows and Honor" series.
Here are some other books from this series: