Good Sister, Bad Sister (first book, Heart of the Staff)

Minuet Dewin, eldest daughter of the wizard Razzmorten, practically raised her half sister Leeuh, who was abandoned by her mother. For many years, Minuet is Leeuh's passionate champion. As time passes, Leeuh becomes increasingly hard to defend as she grows determined to be awful at every turn. Whilst undoing her dangerous pranks, Minuet finds herself the target of her hatred and jealousy. And when they fall for the same prince, it looks like war. Suddenly Leeuh vanishes. She returns years later, compliant and sweet as she always should have been. Minuet is stunned. Should she trust her, or will it be the very death of her?