Murder at the Haunted House by Jim Riley

Twins Mindy and Mandy rely on shaky advice to find ghosts at an abandoned house. Discovered with a bloody knife in their hands, they turn to Hawk Theriot to clear them for the murder of the old man, Gramps Atkins, who once lived there.

Hawk suspects one of Gramps's three sons. Only he does not know which one. All have motive, means and opportunity. After interviewing all three, Hawk reaches a surprise conclusion, despite the continued interference of the twins in the guise of help.

Even the ghosts did not see this one coming.