Love's Magic (Boadicea #1) by Traci E. Hall

While her sisters are tall and beautiful, Celestia Montehue is the misfit in a royal family—petite, with one green and one blue eye. The only thing she has in common with her ancestors is her magical healing ability. Fearing no one will ever accept her, she vows never to marry. Meanwhile, Nicholas Le Blanc, a haunted man, was trained as a knight but his childhood has convinced him he is a bastard. He is captured while on crusade and eventually forced to kill for his freedom. An arranged marriage between the two does not bode happiness, nor does Celestia's new home—a broken-down keep haunted by the ghost of Nicholas' mother. Soon a curse is set upon them, and they must decide if their love will save them or, ultimately, doom them.