For a couple of years they flitted across our screens and newspapers like ghosts. One here, two or three there, once a group large enough to fill a minibus, mostly young, all seduced by a vision of purity, an urge to violence, or both. The Islamic State recruits who travelled from Europe had been neighbours, classmates, colleagues, members of close-knit families and intimate communities before they set off to a war zone, where they planned to build paradise on Earth.

But in their departures, announced on the news every few weeks, they seemed to become insubstantial, unreal, as hard to understand as they were to reach once they had crossed the border into Syria. In Two Sisters, Norwegian journalist and writer Åsne Seierstad, the author of bestseller The Bookseller of Kabul, tries to reverse that process – to turn the fanatical ghosts back into complex humans – by telling in intricate, compelling detail the story of one family dragged into Isis’s web of horror.

The Norwegian-Somali Juma sisters left their Oslo home one October morning in 2013, as if it was any other day. Leila, then aged 16, was ostensibly off to school, while Ayan, three years older and the brains behind their plan, said she was heading across town to see a friend. That evening they emailed their father to say they would never be coming home, that they were going to live in Syria for the sake of their souls and the souls of their parents and three brothers. Two Sisters, then, is also the story of the family shattered by the sisters’ departure.

In her exploration of how and why Ayan and Leila abandoned their home for a distant war, Seierstad weaves a complex picture of their lives as young Norwegians. Ayan’s brilliance at one of the most elite high schools in Oslo, the focus and ambition that she would later apply to religion, shines through in her former teachers’ accounts of her. Tussles with her mother about clothes that were too revealing – before she embraced covering everything but her eyes – the teenage crushes, intense friendships, bands, football and summer camps are the stuff of any adolescence. But Seierstad also teases out the sense of alienation that a distinct and different heritage can produce in a homogeneous society – Ayan’s exploration of her roots and religion, Leila’s isolation at school.

Ayan, particularly, was drawn into a radical milieu just as she reached an age where she was trusted to go out on her own. She was inspired by the charismatic but radical Qur’an teacher who taught many Somali heritage children at weekends, then drawn into the world of IslamNet, a society of young, Salafist Muslims in Oslo. Increasingly, she abandoned homework and spent her weekends trying to win converts at street stands downtown or organising conferences of radical preachers from around Europe.

At home the sisters’ parents, Sadiq and Sara, didn’t monitor the teacher who was secretly radicalising their children, because they trusted his credentials and welcomed their daughters’ new enthusiasm for religion without questioning its intensity.

Security forces failed the sisters. They had only got as far as Sweden when the family first called police for help. Leila was still a minor and it should have been possible to intercept them there, or on their slow passage through Turkey, but their father’s desperate pleas went unanswered for hours.

And teachers, as the sisters voiced increasingly extreme views, broke up classes to pray or skipped school entirely, misunderstood the source of their radicalisation. They worried that Sadiq was planning to marry them off, pressuring them into covering up. In fact, he is a liberal poet who wanted his daughters to have an education and future in Norway.

This collective failing is the reason Sadiq chose to speak out, in the hope that his family’s tragedy might help others to recognise the danger signs. “We were blind,” he says. “We thought it would pass.”

Sadiq set off in search of Ayan and Leila three days after their flight, the start of an ongoing attempt to find them and bring them home. It ruined him financially and nearly cost him his life. At one point he discovered both daughters in the maelstrom of northern Syria, only to be jailed and tortured on the order of Ayan’s husband, Sadiq’s new Isis son-in-law.

For a factual account, some of Seierstad’s descriptions can seem almost too detailed – for example, Sadiq mulling the metallic taste of tea and the play of sunlight on clouds as he flew to Turkey. But Seierstad and Sadiq spent many hours in conversation and the sense of place and time the author conjures makes Two Sisters more than a factual exploration of radicalisation. It is, instead, an unfolding human drama whose characters are compelling – from the strong-willed young women and desperate parents at the story’s heart to the shifting cast that surrounds them – womanising fundamentalists, well-meaning but clueless Norwegian teachers, the loyal Syrian smuggler with close ties to al-Qaida.

Seierstad traces the Jumas’ slow shift to accommodate the once unthinkable, including marriage negotiations with a jihadi hoping to become Leila’s husband. She captures, too, the peculiar language of the young extremists, peppered with Arabic terms and references to God. “Things have been hectic here with Asiyah’s aqeeqah,” Leila writes in a message to Sadiq after her daughter is born, referring to a feast of celebration. “You should know that we are happy and well, we are safe and Allah has provided us with plenty of rizq [gifts].”

Sadiq assumes his son-in-law has written it, but his wife knows better. “That’s how she talks now,” she says, shaking her head.

Seierstad’s wide-ranging book also confronts the tragedy of Syria, revealing how much damage Isis did to the country that was its main base. She explains the complexities of a war that fractured early into factional fighting, and the rise of Isis from the ruins of Saddam Hussein’s Iraqi bureaucracy and the US occupation that followed.

But Leila and Ayan – who start off so human, as Seierstad details their teenage friendships, their lives around Oslo, their progress and battles at school – end up as little more than ciphers. Their characters are as hidden by Isis ideology as their bodies are by the group’s all-enveloping black tents. They did not respond to Seierstad’s interview requests; to family and friends, they are unremittingly upbeat about their lives in Syria.

Their mother – the only relative to whom the girls are still speaking, though there has been no direct contact for a year – never asks them difficult questions. She assumes Isis are listening in and doesn’t want them trapped by their own words.

The horrors of living as a woman under Isis are revealed in Seierstad’s account of Ayan and Leila’s Norwegian friend Aisha, also in Syria, who married a man so violent that he shot her when she tried to leave the house to go shopping, then battered her two-year-old son to death. Still, in her apparently sincere online dispatches, Aisha insisted everything was good in paradise on Earth, and in a chat with a friend, Ayan makes jarring excuses for Isis’s practice of sexual slavery.

Since the book was finished, the Islamic State has collapsed and its fighters and their families have scattered. There is no update on the whereabouts of Leila or Ayan, giving the reader some sense of the void their parents and siblings stare into every day, wondering what happened to them, the two bright girls from Oslo.

• Two Sisters by Åsne Seierstad is published by Virago (£18.99). To order a copy for £16.14 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99