Desert rage : a Lena Jones Mystery / Betty Webb.

Enhanced descriptions from Syndetics:

Ferociously ambitious U.S. Senatorial candidate Juliana Thorsson has been keeping a secret.
The horrific slaughter of a prominent doctor, his wife, and their ten-year-old son brings Thorsson to Private Investigator Lena Jones. The slain family's 14-year-old, Alison, has confessed to the murders. Thorsson wants to hire Lena to discover if Alison is telling the truth, but Lena demands to know what a rising political star wants with a girl she's never met. Desperate for Lena's help, Thorsson reveals her secret - that Alison is the candidate's biological daughter. But that's not all. Thorsson then confides something more than a mere hidden pregnancy, something that could ruin her political plans forever.
Suspecting that Alison's parents had secrets of their own that could have led to the murders, Lena finally accepts Thorsson's assignment. But interviewing those who knew the family well soon puts Lena - now a strong defender of the twoteens - in danger of her life.
Fast-paced, probing, and filled with the trademark twists of the Lena Jones series, Desert Rage once again shows that Betty Webb is unsparing of her characters yet writes their stories with wit and compassion.