Count Me In by Sara Leach:
Twelve-year-old Tabitha is less than thrilled when her parents send her on a hiking trip with her cousins, Ashley and Cedar, and her Aunt Tess. For one thing, she’s not much of a hiker. And she’s pretty sure her cousins hate her. But even Ashley can’t blame Tabitha for everything that goes wrong: the weather turns ugly, a bear comes into the cabin, Ashley and Tess are injured and Max, the family’s beloved dog, disappears. When rescue finally arrives, Tabitha realizes that she is no longer the timid, out-of-shape girl she used to be. She’s become strong, resourceful and brave in the face of adversity–no matter what form it takes.