Winter's Tide by Lisa Williams Kline

This is the story of Diana and Stephanie, new stepsisters on a vacation at a ranch with Diana's mom and Stephanie's dad, who are newlyweds. Diana has a "mood disorder" for which she takes medication. In her estimation, Stephanie is just too perfect. But Stephanie has some secrets of her own. The girls band together to free two captive wolves, an action that has unexpected and unintended consequences. Told in the alternating voices of Diana and Stephanie, the book explores themes of family, friendship, mental health, and nature.