Gimai ga Seijo dakara to Konyaku Haki Saremashita ga, Watashi wa Yousei no Itoshigo desu

Margaret, the duke's daughter, had her engagement broken off by the prince after her powers failed to manifest during the saintly ritual. Rumors spread that her stepsister is a saint instead, but Margaret harbors a secret power: the ability to speak with fairies. Now she's free from the nasty prince