Killer (Alex Delaware #29) by Jonathan Kellerman

The City of Angels has more than its share of psychopaths, and no one recognizes that more acutely than the brilliant psychologist and police consultant Dr. Alex Delaware. Despite that, Constance Sykes, a sophisticated, successful physician, hardly seems like someone Alex needs to fear.

Then, at the behest of the court, he becomes embroiled in a bizarre child custody dispute initiated by Connie against her sister and begins to realize that there is much about the siblings he has failed to comprehend. And when the court battle between the Sykes sisters erupts into cold, calculating murder and a rapidly growing number of victims, Alex knows he’s been snared in a toxic web of pathology.