The Locked Garden by Gloria Whelan

It's the year 1900, the dawn of a new century and a chance for a new beginning for Verna and Carlie, whose mother died two years ago. They are headed to their new home—the grounds of an asylum for the mentally ill. Their father, a doctor, has been hired to treat its patients while the girls are under the strict and watchful eye of their aunt Maude. The towering asylum, the murmuring patients with their tormented pasts, the exquisite locked garden at the center of the grounds—Verna perceives forbidden mystery and enchantment everywhere. Even Aunt Maude's temper will not keep her from striking out on her own exciting adventures.

But is Verna ready to confront all the secrets and emotions that have been locked within—even those of her own heart?