A Sword Upon the Rose by Brenda Joyce

A bastard daughter, Alana was cast away at birth and forgotten by her mighty Comyn family. Raised in solitude by her grandmother, she has remained at a safe distance from the war raging through Scotland. But when a battle comes close to home and she finds herself compelled to save an enemy warrior from death, her own life is thrown into danger.
Iain of Islay’s allegiance is to the formidable Robert Bruce. His beautiful rescuer captures both his attention and his desire, but Alana must keep her identity a secret even as she is swept up into a wild and forbidden affair. But as Bruce’s army begins the final destruction of the earldom, Alana must decide between the family whose acceptance she’s always sought, or the man she so wrongly loves.