Sacred and Profane - Faye Kellerman

Los Angeles Police Detective Peter Decker had grown very close to Rina's young sons, Sammy and Jake, as he had to their mother, and he looked forward to spending a day of his vacation camping with the boys. A nice reprieve from the grueling work of a homicide cop-until Sammy stumbles upon a gruesome sight...
Two human skeletons, charred beyond recognition, are identified by a forensic dentist as teenage girls--and for Decker, the father of a sixteen-year-old daughter, vacation time is over.