Turkey said it had arrested 65-year-old Rasmiya Awad in northwest Syria, with an official describing it as an intelligence "gold mine".
She was detained in a raid on a trailer container where she was living with her family.
She was with her husband, daughter-in-law and five children when she was captured near the town of Azaz, which is part of the region administered by Turkey.
The adults are being interrogated, the official said.
Little is known about al Baghdadi's sister.
