Turkish authorities said they captured the elder sister of dead ISIS leader Abu Bakr Al Baghdadi on Monday in the northern Syrian town of Azaz.
Officials are interrogating her husband and daughter-in-law, who were also detained in the raid.
Rasmiya Awad, 65, was captured in a Turkish-controlled Syrian town near the border, a senior official told Reuters. She was with her five children.
The town is part of the region administered by Turkey after it invaded Syria.
Allied Syrian groups manage the area known as the Euphrates Shield Zone.
Not much is known about Awad, but Turkish officials said she was affiliated with ISIS.
“This kind of thing is an intelligence gold mine. What she knows about (IS) can significantly expand our understanding of the group and help us catch more bad guys,” an official said.
The Syrian Observatory for Human Rights confirmed the arrests.
"Turkish forces arrested the sister of Baghdadi last night in a camp in the suburbs of the city of Azaz", said the head of the organisation, Rami Abdel Rahman.
"They arrested her, her husband, her daughter-in law, and her five grandchildren."
Al Baghdadi blew himself up last month after he was cornered in a tunnel during a raid by US special forces in north-western Syria.
The raid was a major blow to ISIS, which has also lost the territory it held in Syria and Iraq.
ISIS vowed revenge against the US for the death of its leader in an audio tape posted online on Thursday.
Al Baghdadi's aide was killed hours after the raid, also in north-western Syria, in a US strike.
Al Baghdadi rose from obscurity to lead the terrorist group, holding sway over large areas of Iraq and Syria from 2014 to 2017 before control was wrested away by US-led coalition forces, including Iraqis and Syrian Kurds.
The terrorist group said a successor to Al Baghdadi, Abu Ibrahim Al Quraishi, had been appointed.
World leaders welcomed Al Baghdadi's death, but they gave a warning that the group, which carried out atrocities against religious minorities remained a security threat in Syria and beyond.