Heavily pregnant with her third child, Shamima Begum, now 19, was tracked down by The Times to a refugee camp in northern Syria.
Ms Begum’s elder sister Renu told ITV News: "I’m so relieved that my sister has been found, safe and sound," adding the family would like her come home.
But will authorities let the teenage runaway return to Britain?
Here are some key questions her case raises:
What action might she face if she is allowed to return to the UK?
Home Office Minister Ben Wallace has said everyone who returns from taking part in the conflict in Syria or Iraq must expect to be investigated by the police.
This will determine if they have committed criminal offences, and to ensure that they do not pose a threat to national security.
Ruling out an operation to bring Ms Begum back to the UK, Home Office Minister Ben Wallace has said that the British government will not "put at risk," the lives of British civil servants to bring home "terrorists or terrorist supporters."
Advice from the Foreign and Commonwealth Office advises British citizens not to travel to the war-torn state. The minister said: "Anyone who does travel to these areas, for whatever reason, is putting themselves in considerable danger."
He added that people who take part in the Syrian conflict can "expect to be investigated by the police to determine if they have committed criminal offences, and to ensure that they do not pose a threat to our national security."
Those guilty of wrongdoing can be prosecuted under a range of terror laws for offences committed overseas. Authorities have the power to use exclusion orders to bar them from returning to Britain.
What have Ms Begum's family said about her coming back to the UK?
"She's pregnant and vulnerable, and it’s important we get her out of al-Hawl camp and home as soon as possible.
"We hope the British Government will help us bring her home to us where she belongs," she said.
Speaking to The Telegraph, the father of Shamima Begum said the family has been "left distraught," by his daughter's decision to travel to Syria.
Since her disappearance, Mohammad Nizam Uddin said he had been unable to reconcile himself. The 42-year-old said: “We have heard nothing from her since she left. We do not know where she is.
“As a father I urge the British Government to let these girls back into the country. Please let them come back. I want to see my daughter again. It is terrible she is not here, it is terrible for us.”
He added: “I think they should be allowed to come home. When they went to Syria they were not mature and they had been radicalised.”
How does her former legal representative view her return?
Tasnime Akunjee is the former lawyer of Ms Begum. Speaking to ITV News, he said: "she was a schoolgirl when she left and she found herself in a situation which is very difficult to come away from, it has taken some years for her to get to this point. We'd hope that she is treated as a victim."
Mr Akunjee believes the British government should assist Begum with her return to the UK. He said: "I don't imagine we have a government of such snowflakes that they can't walk into a refugee camp which is perfectly safe."
Whitehall has said that it will not embark on such an operation due to security concerns.
How should Ms Begum be treated by UK authorities?
Ms Begum was a pupil at Bethnal Green Academy in east London when she ran away to Syria in February 2015 with Kadiza Sultana and Amira Abase, who also attended the school.
Tasnime Akunjee, a lawyer who was instructed by the girls’ families after they disappeared, said that that British authorities should be reminded of former Metropolitan Police Commissioner Bernard Hogan-Howe's position at the time.
Lord Hogan-Howe said that the girls would be treated as victims so long as they had not committed any further offences while in Syria.
How many British women and girls joined IS and how many have returned?
An absence of public government data has led experts to warn the number of women and minors linked to IS thought to have returned has been significantly underestimated.
An estimated 850 people have travelled from the UK to support IS in Iraq and Syria, including 145 women and 50 minors, according to a report by King's College London released last year.
Of the 425 who have returned, only two women and four minors were listed.
Do they pose a threat?
Joana Cook, a senior research fellow at KCL who co-authored the study, said the researchers considered the women to be potential security threats based on several factors.
She said: "(These include) the physical security roles and related training that women have undertaken in IS-held territory, and the potential to transfer or apply these skills in other locations, or to pass these on to other people… including other women and their children."
One woman who left the UK for Syria has " for the decision. She claims she made to move to offer her skills to assist the dire humanitarian situation brought about by Syria's civil war, but now wishes to come back to the UK.
What is left of Islamic State?
When Ms Begum left for Syria in 2015, IS had taken control of large tracts of northern Syria and northern Iraq.
Millions of people were living in territory controlled by the terrorist group, which had declared the creation of a caliphate.
Four years later, following a massive military campaign by regional forces assisted by foreign powers including America, Britain and Russia, IS territory has dwindled to a small village in eastern Syria.
It's a far cry from the area the organisation previously controlled. At one point it claimed to hold an area equal in size of Britain.
Earlier this week, US-backed Syrian forces launched an operation to clear Baghuz where, according to Ms Begum, Ms Abase and a fourth Bethnal Green schoolgirl, Sharmeena Begum, remain
What have Counter Terrorism Policing said?
A Counter Terrorism Policing spokesman said: "The threat posed by UK nationals seeking to return from Syria or Iraq is something we have planned for and a wide range of measures and powers are being used to manage and mitigate that threat.
"Anyone who returns from Syria or other conflict zones, having gone in support of any proscribed terrorist group - whether that’s fighting for or against Daesh, can expect to be investigated by the police.
"This includes individuals suspected of committing the most serious offences, such as bomb attacks, shootings and the murder and attempted murder of civilians and Coalition security forces."
They continued: "Any investigation is carried out with an open mind and based on evidence available. People can be prosecuted in the British courts for terrorist activity in Syria or elsewhere in the world."
What about the other British people who left the UK to join IS?
Six other British women are in the same camp as Ms Begum. Between them they have 13 children.
Speaking to ITV News, one of the women expressed her desire to return to the UK, primarily so that she is able to educate her children. She said that she would not "give up" her children if they were allowed to return to the UK without her.
She said: "I would like the British government to help us, to know that we are here and we want their help."
The woman added that she thinks Islamic State is "finished" after months of bombardment by an international group of allies set on defeating it.