The Princess Wars — Melody Carlson

The twin princesses of Raspen Kingdom have been at odds for their entire lives. Firstborn Heather is a no-frills girl who grew up riding horses, fencing, and dreaming of great adventures. Meanwhile her twin sister Rose loves ornate jewels, silk gowns and being treated like royalty. Because the king is sickly, Heather is due to take the throne on the twins' eighteenth birthday. But before she is crowned, Princess Heather embarks on an exciting journey throughout the kingdom. And it's not long before Heather's adventure goes sideways and threatens to undo the entire kingdom.