She Who Would Be King by Kim Pritekel

Cateline, the seventeen-year-old daughter of a nobleman in fourteenth-century France, lives in a time when children are used as pawns and bargaining chips. She is married off to Fergus, prince of Sursha, a Gaelic-speaking island nation near Ireland, who is next in line when the beloved King Carthac dies. Or is he? Fallon, the youngest royal child and only girl, has been raised as one of the king's sons for reasons she has never understood. A natural fighter, she heads the Crown's Elite Guard protecting her boorish brother. When Fallon is forced to stand in for Fergus, an instant attraction forms between her and Cateline. In a game of thrones filled with deception and betrayal, even the most secret love can mean death.