The North East Counter Terrorism Unit confirmed the Dewsbury teenager was questioned in April under section 5 of the Terrorism Act and later bailed.
Hassan Munshi and Talha Asmal, both 17, are feared to have travelled to Syria to join Islamic State in March.
West Yorkshire Police and counter-terrorism officers are investigating.
Speaking at the time of their disappearance, the boys' families said they were praying for the safe return of the "ordinary Yorkshire lads".
Hassan's brother, Hammaad Munshi, was arrested in 2006 at the age of 16 after police found a guide to making napalm on his computer.
He became the youngest person to be convicted under the Terrorism Act.
A 17-year-old boy has been arrested on suspicion of preparing terrorist acts as part of a search for two teenagers from Dewsbury thought to be in Syria.