Obsession (Alex Delaware #21) by Jonathan Kellerman

Tanya Bigelow was a solemn little girl when Dr. Alex Delaware successfully treated her obsessive-compulsive symptoms. Now, at nineteen, she still seems older than her years–but her problems go beyond hyper-maturity. Patty Bigelow, Tanya’s aunt and adoptive mother, has made a deathbed confession of murder and urged the young woman to seek Delaware’s help.

The doctor recalls Patty as a selfless E.R. nurse struggling to raise a child on her own–a woman seemingly incapable of the “terrible thing” she has admitted. But for Tanya’s peace of mind, Delaware agrees to investigate, and he enlists LAPD detective Milo Sturgis in the search for the phantom victim of a crime that may never have occurred.