- "Blood is thick as thieves."
- — Tag-line, from front cover
Blood Relations is the 15th book in The Hardy Boys Casefiles series. It was first published by Archway Paperbacks (an imprint of Simon & Schuster) in May 1988.
From end of Casefile #14 -- Greg and Mike Rawley are the new boys at Bayport High. Adopted by the wealthy Walter Rawley, the boys fit in just a little too well. Rivalry grows between them and the Hardys when Callie Shaw grows fond of Greg, and Mike is a challenge to Joe on the football field. Then Greg and Mike's mother, Linda, discovers the diary of Rawley's first wife. The diary leads Linda Rawley to suspect her mild-mannered husband of being a spy, or even a murderer. Did the late Mrs. Rawley really die of natural causes? Fearing for their mother's welfare, Greg and Mike turn to the Hardys for help. And when Linda Rawley is kidnapped, Frank and Joe pose as her sons. Could this turn into a very lethal family affair?
From back of book -- Frank and Joe Hardy are enjoying time off when the two new boys in town, Greg and Mike Rawley, convince them to investigate their stepfather, Walter. Greg and Mike suspect him of plotting to kill their mother, Linda. But there's a hitch: Walter Rawley is one of Fenton Hardy's oldest friends and an upstanding citizen of Bayport. With the Rawley boys' help, the Hardy brothers check out the contents of Linda Rawley's safe, looking for the diary of Walter's first wife, which contains the murderous evidence. Then Linda is kidnapped. Using themselves as bait, the Hardy boys flush out the kidnappers -- only to find they are mere pawns in an elaborate scheme against their father's old friend.