Briarheart by Mercedes Lackey

From a beloved fantasy author comes this fresh feminist retelling of Sleeping Beauty about one girl destined for greatness--and the powerful sister ready to protect her by any means necessary.

Miriam may be the daughter of Queen Alethia of Tirendell, but she's not a princess. She's the child of Alethia and her previous husband, the King's Champion, who died fighting for the king, and she has no ambitions to rule. When her new baby sister Aurora, heir to the throne, is born, she's ecstatic. She adores the baby, who seems perfect in every way. But on the day of Aurora's christening, an uninvited Dark Fae arrives, prepared to curse her, and Miriam discovers she possesses impossible power.

Soon, Miriam is charged with being trained in both magic and combat to act as chief protector to her sister. But shadowy threats are moving closer and closer to their kingdom, and Miriam's dark power may not be enough to save everyone she loves, let alone herself.