Chaimaa Amghar, 20, was arrested in the Brussels district of Molenbeek on September 21 after police traced social media messages indicating she was attempting to "commit acts of terror-linked crimes in a conflict zone", the source said.
The suspect is the sister of Sofiane Amghar, one of two suspected Islamic State members killed in a raid in the eastern city of Verviers in January 2015.
Prosecutors later said the group was was under the orders of Abdelhamid Abaaoud, the ringleader of the terror cell that attacked Paris on November 13, 2015.
Theu then slipped back into Belgium to the Verviers hideout.
Authorities have said the November Paris and March Brussels attacks were the work of the same cross-border jihadist group with deep roots in the Belgian capital.