Double Exposure (The Hardy Boys Casefiles #22)

"A long-lost brother is always a mystery.

When Frank and Joe decide to meet their father's latest contact on their own, they get more than they bargained for. The young detectives come face-to-face with their long-lost brother, Chris Hardy! But a hail of bullets cuts their meeting short.

Suddenly Frank and Joe find themselves trapped in a deadly cross fire between a brutal gang of spies and a high-tech computer scam. Odds are their new brother is the triggerman -- aiming to wipe out an old family debt."

(Hardy Boys Encyclopedia 1988)