At the height of Islamic State’s rule, Umm Sayyaf would regularly prepare her home in eastern Syria to receive her husband’s friend. When the bearded man in the black robe arrived she would make tea and lay a platter of food in a sitting room. Other doors in the sprawling house in the town of Shahadah remained locked; enslaved women and girls from the Yazidi community huddled together in one room, and an American hostage, Kayla Mueller, sat alone in another.

The man in the black robe was Abu Bakr al-Baghdadi, the leader of Isis, and Umm Sayyaf’s husband, Abu Sayyaf – an extremist who had risen through the ranks of the terror group alongside Baghdadi to become one of its top three figures – was the Isis oil minister. She, meanwhile, came from a well-known Iraqi jihadist family: her lineage and spouse earned trust and made her one of the few women to have regular access to the Isis leadership – until the day US commandos came for them in May 2015.

In the four years since the raid that killed her husband and led to her capture, Umm Sayyaf, 29, otherwise known by her birth name, Nisrine Assad Ibrahim, has been the most important of Isis wives in captivity; a keeper of some of the organisation’s darkest secrets and an alleged participant in some of its most depraved acts.

One damning charge has remained central to her infamy: that she acted as jailer and enforcer over Mueller, a humanitarian worker who had been captured by Isis 13 months earlier and was raped by Baghdadi in the Sayyafs’ home. Another is that she enslaved and brutalised nine Yazidi women and girls who had been captured in Iraq and brought to the Isis leaders. Umm Sayyaf has also been charged by the US government with being a party to a terror conspiracy that led to Mueller’s death in Raqqa in February 2015.

In her first interview, Umm Sayyaf painted a picture of the Isis leadership that reaffirmed stories told by victims of the group’s savagery. She told of the desperate plight of Yazidis who were brought to the house, how their arrival stirred up animosity between Isis women and their husbands and how Mueller had been seen as both a high value prize for Isis and a personal possession of Baghdadi.

Abu Bakr al-Baghdadi in a picture from 2014. Photograph: AP

She spoke to the Guardian, partly through a translator, at a prison in Erbil, Iraq, where she has been held since a court in the city sentenced her to death. She was accompanied by a Kurdish intelligence officer who made no attempt to intervene in the interview.

Umm Sayyaf also described herself as innocent party, who had no choice but to follow orders. Her self-portrayal is vehemently disputed by Mueller’s parents, Marsha and Carl Mueller, who maintain that Umm Sayyaf was an architect of their daughter’s suffering – a belief they say has been confirmed by accounts of Yazidis who were held captive with her.

As Isis swept through eastern and northern Syria from April 2013, subverting communities, killing at will and sending hundreds of thousands fleeing, the group kidnapped up to three dozen foreigners, whose plight was to play out globally over the next two years. Mueller was among them.

She had crossed into Syria with a friend to help install internet equipment in an Médecins Sans Frontières-run hospital in Aleppo in August 2013 and had been kidnapped as she was leaving after an overnight stay. For the next 13 months, before being sent to the Sayyafs home, Mueller was transferred to several prisons, one of which was a mill outside of Raqqa where the largest group of foreigners was held.

Umm Sayyaf corroborates claims that Mueller, from Prescott in Arizona, was brought to the property in Shadadah by Abu Sayyaf around 24 September 2014. At about the same time, he also brought the Yazidis. “The Yazidi girls changed not only my life, but the lives of all the girls in the Islamic State,” she said from a meeting room inside a counter-terrorism prison in Erbil. “Our husbands became like wild animals when they were around. They had no respect for us.”

“But she was treated differently from the Yazidis,” Umm Sayyaf said of Mueller. “There was a budget for her. Pocket money to buy things from the shop. She was a lovely girl and I liked her. She was very respectful and I respected her. One thing I would say is she was very good at hiding her sadness and pain.”

Up to 3,000 Yazidi women were enslaved by Isis when its members overran their northern Iraqi communities in August 2014. Many were passed around between Isis members who raped and abused them.

Throughout the 90-minute interview, she insisted that Mueller was considered by both herself and her husband to be “better” than the Yazidis. “Baghdadi had told her if you become a Muslim, you will be free. He stayed with us twice in that month. Once he was coming from Raqqa to Iraq, and the second time from Iraq to Raqqa.” She reluctantly admitted that the young American had been “owned” by Baghdadi, but claimed she did not know what that entailed.

Kayla Mueller. Photograph: Family

The stance conflicts with admissions she made in interrogations with her jailers in Erbil and with the FBI, both of whom allege she was tasked with preparing Mueller for whenever Baghdadi arrived. Her claim to have treated Mueller kindly is also at odds with what several Yazidi girls who escaped the home later told Mueller’s parents. According to their accounts, Umm Sayyaf was as brutal and tyrannical with Mueller as she was with them.

More than four years after their daughter’s reported death in 2015, the Muellers remain deeply angered by what they believe was a slow and unhelpful response from the Obama administration to their daughter’s situation, which denied them the tools to negotiate with Isis effectively.

“What we have learned, we have learned from the brave Yazidis who were held captive with Kayla,” said Marsha Mueller, from the family home in Arizona. “From what we understand Kayla was treated harshly, like the Yazidis and like the western hostages. We learned from the young Yazidi girls held with Kayla that she suffered terribly, but that she tried to hide her own pain. They said Kayla was like a mother or a sister, always trying to protect them and encouraging them that one day they would get home.

“They made it sound as if Kayla put herself in harm’s way to protect them and she suffered physical and emotional retaliation for this. They told us … Kayla was writing her family memories and would share them with the girls. Kayla was only with the girls that escaped for approximately six weeks, but they seemed to care deeply for each other. They claimed Kayla as their own and I believe Kayla claimed them as well.”

Carl and Marsha Mueller hold candles at a memorial for their daughter in 2015. Photograph: Rob Schumacher/AP

Kurdish officials believe the introduction of the Yazidi captives angered the senior Isis wives and helped crack the group’s watertight secrecy: “They tried to justify what they were doing [raping the women and girls] through the sharia,” a senior Kurdish intelligence official said. “But, deep down, the wives never forgave them. And that played a part in their downfall.

“[Umm Sayyaf] said to Abu Sayyaf: ‘I love you and I don’t want you to be with the Yazidis.’ He said: ‘If you don’t like it, you can leave.’ All that support, and she didn’t mean anything to him. We have used this leverage quite a lot and they have been very useful to us.”

Umm Sayyaf said Mueller busied herself with learning while in Shahadah. “She was all the time reading and writing, every time I saw her she was busy with books. When I was cooking, she came to me and said: ‘Teach me what you are doing.’ Abu Sayyaf told her not to ask me any questions. I asked her how they captured her. We used to call her ‘Iman’. Once I asked her what her real name was, and she replied ‘Kayla’.

Now on death row, Umm Sayyaf says she last saw Mueller in late 2014, when Baghdadi arrived from Iraq. “He took her with him in a simple car, a Kia. He was driving, and they went to Raqqa.” Three months later, when she and Abu Sayyaf were also in the Syrian city, she saw a news report about Mueller’s death. “I went and asked my husband about it. He looked as shocked as I was.”

How Mueller died remains disputed. Isis claims she was killed in an airstrike launched to avenge the public burning to death of a captured Jordanian pilot days earlier. The US government denies a strike took place at the time, and says Isis killed her. The family received three photos of what was purported to be Mueller’s body, in the last of 27 emails they exchanged with the terror group. The fruitless correspondence still stings deeply.

“Isis kept indicating that they were willing to release Kayla, but of course there had to be some negotiation and the administration at that time kept sending [clear] messages that they were not interested [in negotiating with Isis],” said Marsha Mueller. “They were not truthful with us, only when their hand was forced by damage control did they tell us what we needed to have known if we were to act effectively on behalf of our Kayla to secure her release and keep her from the horrific torture and suffering she endured.

“Can you imagine? Our own government who I trusted completely to help us, who we were given every detail of what we knew, took what we gave them to use, but did not share with us what was happening to our daughter.

“We feel that they used Kayla and us simply for their own motives to get Baghdadi and score a political victory. We believe Kayla was taken by al-Baghdadi to rape and torture because it provided a powerful political statement that he, not the US, had the upper hand. Kayla was used differently because she was a high-value political prisoner, but she was still a prisoner – their prisoner – and she suffered in ways we believe were avoidable if the administration in office at that time had been honest with us.”

The Muellers also direct criticism at MSF: they say Isis had passed on email details to its staff through which the family could begin negotiations. “But MSF withheld [for several weeks] the information with which Isis had provided them … When we received the address from MSF, within hours we had an email from Isis.”

MSF said it withheld the email details out of fear for the safety of other hostages.

Two months after Mueller’s apparent death, the US military finally caught up with the Sayyafs, at an oil field about 20 miles (30km) from Shahadah. One of the Yazidis brutalised at their home had made it to Erbil and passed on details of where the couple were staying. “I heard the helicopters, and my husband told me to put on my hijab,” she recalled. “We were going downstairs and they shot him. I was blindfolded and put on the helicopter.”

As well as the death sentence, Umm Sayyaf is also facing efforts to transfer her to the US on terrorism charges.

The Muellers remain determined to learn the complete truth of their daughter’s captivity, and to secure her legacy. “Kayla wouldn’t want us to be concerned only with what happened to her,” said Marsha Mueller. “The world has to come together and do better by these victims of genocide and terrorism and seek justice not just for the western hostages but for the brutal torture of the Yazidis.”