Looking back from old age, Lucy Watson describes the little sister that only she really loved—``our Sarah,'' who was born when Lucy was six and disappeared a few years later, while Cromwell's soldiers were destroying churches all over England. Slow of speech and dauntingly unpredictable, Sarah was thought by many to be a changeling; when she vanished just at the time her mother died in a later childbirth, her own father named the new baby Sarah in the hope that she was his own real child returned at last. Only Lucy believed that the first Sarah had simply wandered off and might be alive; and though Lucy never saw Sarah again, she eventually learned that this was indeed what happened, and that Sarah found others to love and take her in before straying yet again. With carefully chosen detail expertly woven into her narrative, Hersom brings the life of the common people in the Yorkshire Dales of the 17th century vividly to life; her style, colored by local expressions yet easily understood (especially with the help of the short glossary), is lively and consistent, with the pleasing cadence of fine storytelling. An enriching, enjoyable mystery with an unusual cast of well-realized characters. (Fiction. 11-15)