The Jasmine Throne (The Burning Kingdoms Book 1)

Modern Mrs Darcy team member Leigh mentioned this in WSIRN episode 291: The best books of summer (so far) as one of her favorites. From the publisher: "A ruthless princess and a powerful priestess come together to rewrite the fate of an empire. Exiled by her despotic brother, princess Malini spends her days dreaming of vengeance while imprisoned in the Hirana: an ancient cliffside temple that was once the revered source of the magical deathless waters but is now little more than a decaying ruin. The secrets of the Hirana call to Priya. But in order to keep the truth of her past safely hidden, she works as a servant in the loathed regent’s household, biting her tongue and cleaning Malini’s chambers. But when Malini witnesses Priya’s true nature, their destinies become irrevocably tangled. One is a ruthless princess seeking to steal a throne. The other a powerful priestess desperate to save her family. Together, they will set an empire ablaze."