The Queen of Ieflaria - Tales of Inthya, #1

Princess Esofi of Rhodia and Crown Prince Albion of Ieflaria have been betrothed since they were children but have never met. At age seventeen, Esofi's journey to Ieflaria is not for the wedding she always expected but instead to offer condolences on the death of her would-be husband. But Ieflaria is desperately in need of help from Rhodia for their dragon problem, so Esofi is offered a new betrothal to Prince Albion's younger sister, the new Crown Princess Adale.
But Adale has no plans of taking the throne, leaving Esofi with more to battle than fire-breathing beasts.