Trickster's Choice by Tamora Pierce

Alianne of Pirate's Swoop wants to be a spy.  Her father is a spymaster and her mother is Alanna the Lioness, The King's Champion and a Lady Knight.  Although her parents want Aly to accept responsibility as a noblewoman and get a job, neither of them want her to spy, as it is dangerous work.  When Aly and her mother get in an argument about her job choice, Aly decides to leave for a family friend's house so her mother and father can have some alone time while Aly cools off.  Unfortunately, Aly is captured by pirates and given away as a slave to the Baltaings.  When the king exiles the Baltaings for the time being, Aly goes with thanks to the trickster god, Kyprioth.  Kyprioth sets a wager with Aly.  If she can keep the two eldest Baltaing daughters alive and safe until the autumn equinox, he will send her home and convince her father to let her spy.  Aly agrees and works hard at her job while also obtaining lots of information for her father.  This is Aly's chance to prove to her parents that she can handle the dangers of being a spy and she's determined to make it work, no matter how deep the plot of the royal family may go.