Blood Is Thicker Than Wine by Liz Eagle

After her father's death, Lil Starling, a resourceful federal probation officer, discovers information hidden in dusty old carboard boxes that recounts the only double homicide that took place in her home town of Warsaw, NY. She is shocked to learn that her grandfather's handwritten notes seem to implicate her own father and her relatives in the double murder that is over 75 years old. She gradually goes through all the boxes that she was handed as part of her father's estate, but enlists the help of her friend, a federal judge, to help her find out who was responsible for the murders. All the players are deceased, which makes the task even more difficult, but now, after her father's death, the probation officer and judge combine their resources to get to the bottom of the case. The closer she gets to determining the real killer, the more her life is in danger.