Khairat el-Shater's son-in-law Ahmed Tharwat Abdel-Hamied was arrested on Saturday at Cairo International Airport upon his arrival from Qatar, a senior security source told MENA.
Khairat el-Shater, now in prison, is the deputy supreme guide of terrorist Muslim Brotherhood (MB) group.
Abdel-Hamied was on the airport watch list, he added.
The Homeland Security intercepted communications between the suspect and members of the Muslim Brotherhood's international organization, the source said.