A Turkish official said she was captured during a raid today near the town of Azaz along with her husband, daughter-in-law and five children.
Little is known about the sister of al-Baghdadi, who was killed in a US raid in the nearby province of Idlib last month.
The area she was found is part of Syria that Turkish forces invaded last month following the withdrawal of US troops from the region.
The adults taken into custody by the Turks are being interrogated, the official said, speaking on condition of anonymity In line with government protocol.
'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,' the official said.
Last August a different sister, whose identity was not released by authorities, was sentenced to death in Iraq.
She was found guilty of 'offering logistic support and help to [ISIS fighters] in carrying out criminal acts', according to reports at the time.
Al-Baghdadi was cornered by Delta Force commandos as they stormed his compound in the village of Barisha near Idlib, northern Syria, on October 26.
The extremist cleric blew himself up by detonating a suicide belt after running into a dead end and dragging two of his children with him to their deaths.
The raid was a major blow to ISIS, which has lost territories it held in Syria and Iraq in a series of military defeats by the US-led coalition and Syrian and Iraqi allies.
Many IS members escaped through smuggling routes to north-western Syria in the final days of battle ahead of the group's territorial defeat earlier this year, while others have melted into the desert in Syria or Iraq.
The reclusive al-Baghdadi, who is Iraqi from Samarra, was known to be close to one of his brothers, known as Abu Hamza.
Al-Baghdadi's aide, a Saudi, was killed hours after the raid, also in north-western Syria, in a US strike.
The group named a successor to al-Baghdadi days later, but little is known about him or how the group's structure has been affected by the successive blows.
Intelligence on his whereabouts also came from Iraqi forces who captured members of Baghdadi's inner circle, including his courier and the courier's wife.
Last week the Pentagon released footage showing the raid that resulted in the death of al-Baghdadi.
An audio tape posted online on Thursday, confirmed that its leader had died and vowed revenge against the United States.
Baghdadi had risen from obscurity to lead the ultra-hardline group and declare himself 'caliph' of all Muslims, holding sway over huge areas of Iraq and Syria from 2014-2017 before Islamic State's control was wrested away by US-led coalition forces including Iraqis and Syrian Kurds.
The group, also known as ISIS, said a successor to Baghdadi identified as Abu Ibrahim al-Hashemi al-Quraishi had been appointed.
A senior US official last week said Washington was looking at the new leader to determine where he came from.
World leaders welcomed his death, but they and security experts warned that the group, which carried out atrocities against religious minorities and horrified most Muslims, remained a security threat in Syria and beyond.
This article has been adapted from its original source.