Birmingham, 1980. British-born Zana Muhsen was 15, and her sister Nadia was 14. It was summer, and the girls' father, Yemen-born Muthan Muhsen, promised them a six-week holiday with their Yemeni relatives. The stories he told them of the Yemen's sunshine, camel-riding, palm-fringed beaches, and sand dunes must have made surreal contrasts to the simple life they were living above a fish and chip shop in working-class Birmingham. The young teenagers couldn't wait to start the holiday.

It was only when they arrived in the Yemen that they discovered their father had sold each of them into marriage to 13-year-old boys, for $2,500 apiece. For eight years, the sisters lived as peasants' wives in a remote mountain village, where a medieval way of life still existed and where there was virtually no contact with the developed world.

In 1987, after a British-based campaign by the girls' mother, Miriam, the Observer sent a reporter and a photographer to the Yemen to find the sisters. The subsequent story was front-page news, and as a result of the ensuing publicity, Zana was allowed to leave by the authorities, on condition she left her son behind as they would not issue him with a passport. Nadia, who had two children at that time, stayed behind, hoping that her children would all get passports shortly. She is still there.

In 1990, Zana co-wrote Sold: One Woman's True Account of Modern Slavery, which initiated another flurry of interest in Nadia's case. But Nadia herself seems to have become a pawn in international bureaucracy: the sisters were defined as "dual nationalities", and neither Britain nor the Yemen was prepared to lose diplomatic face on the matter.

Twenty years after Nadia Mushen first went to the Yemen, it is not known whether she remains there against her will or whether she has a will left at all. Her sister and mother remain convinced that she wants to return to Britain, and they have been campaigning to that end for the past 12 years.

Zana now has three children and still lives in Birmingham. She has written a second book, A Promise to Nadia. The promise of the title is one Zana made to her sister before she said goodbye to her: that she would not leave her there. The book updates the story of Zana's life so far, and of the efforts over the years to bring her home: the book's focus is very much on Nadia, and not Marcus, the child Zana had to leave behind. Zana felt that, as a male, he would fare better in the Yemen's patriarchal society than a female.

It is all very dark stuff. In their Dublin hotel room, Miriam and Zana wear their lives on their faces. They look haunted and exhausted with the paralysing obsession of trying to reclaim their daughter and sister to their lives. "I never forget about her, not for any part of any day," Zana says. "I cry every single day."

Miriam just stares out the window at the seagulls, and every now and then she comes out with a phrase about Nadia that suggests she is constantly having a running monologue about it all in her head, and that these phrases are mantras she repeats over and over again. "She wants to come home. They make her say things she doesn't mean. Her teeth are falling out. She has six children now. She wants to come home. She wants to come home."

Miriam has had two nervous breakdowns lately, and now has agoraphobia.

The expression of blank suffering on her face is a disturbing contrast to the old-fashioned opulence of their hotel room.

After 20 years, do they not think that if Nadia was to come back, she would be culture-shocked and traumatised?

"She will be coming back to her own culture," Zana says firmly. "She was only 14 when she went out there. She's still a child in her mind. Her mind is very small. It hasn't opened since she's been away; how could it? There is no news out there of anything happening in the outside world. I am her voice now. That's why I'm fighting for her.

"She keeps giving the same robotic answer to us: that she wants to come home someday but not yet. They are making her say that. People don't understand how traumatised you become after years of being raped and being forced to do all that physical labour and being isolated from the outside world. She is like she's dead."

They last spoke to Nadia on the phone four years ago, on her birthday. Nadia did not know how old she was: she got her age wrong by a year. There is a tight-knit Yemeni community in Birmingham, who are aware of the Muhsen sisters' case. "They admit what my dad did was wrong, but they have never helped us in any way."

Do the women think that because they were Islamic and working-class, that the politicians and diplomats have been slow to press their case over the years? They reply simultaneously.

"My name went against me," says Miriam. "When I rang to say my daughters had been abducted, they just heard my name and thought it was all part of our culture."

"It was the colour of my skin," says Zana, in her distinctive Brummie accent. They hope Zana's new book will stimulate public opinion, and that this will put pressure on the authorities to move Nadia and her children to Britain.

"All the money from this book, it's going into an account for Nadia when she comes home," Zana states. "She'll need dental treatment. Her teeth are falling out. Her teeth are falling out," Miriam incants, in a moment that could have come from Kafka or Beckett, but which is neither: they are real words that suggest profound despair.

A Promise to Nadia: the True Story of a British Slave in the Yemen, by Zana Muhsen with Andrew Crofts, is published by Little Brown, at £9.99 in UK

More information on the Muhsen case, including petition details, is available on: web.infiniweb.ca/nadia