Josef Fritzl trial will be held with key figure absent: his wife Rosemarie

The dumpy housewife who stood by him for decades is not accused in Austria's trial of the decade. There is no apparent prospect of her being prosecuted. But according to one of the very few people who still speak to her, her sister Christine Renner, she is suffering for her husband's crimes in her lonely council flat, not far from the old family home in the tiny town of Amstetten, population 23,000.

"My sister Rosemarie has really been destroyed by this whole business," Mrs Renner told The Sunday Telegraph. "She's had a terrible life and I never thought I would say it - but I think her life is even worse now than it was when she was with him.

"As well as losing the children, everyone thinks she must have known something."

Detectives hastily declared that she didn't after they interviewed her last April - as an astonished world learnt of Elisabeth, the daughter Fritzl abused, and the children he fathered with her who had emerged from a secret cellar of the suburban home.

Police investigated a claim that Mrs Fritzl was sometimes spotted in supermarkets buying nappies and baby food with her husband, but she insisted she never had any idea of what had been going on under her feet for 24 years. She has never been subject to a full investigation by state prosecutors.

Yet her account of life with her husband implies an extraordinary degree of naivety on her part in not realising what was going on.

In 1984 when Elisabeth vanished Fritzl told his wife that their daughter had run away to join a cult, she told detectives. His almost nightly absences in the cellar where she was forbidden to go were "engineering work", he told her. She had never heard a thing from down there, apparently.

And when her husband presented her with three babies to raise, he explained that they had been dumped by the missing Elisabeth on a doorstep with a note, she told police. In fact they were the progeny who were too noisy to keep in a cellar.

Now she is divorcing her husband, but she stood by him in 1967 when he was jailed for raping a nurse at knifepoint, and she is alleged to have told police that her husband "sexually attacked" Elisabeth when she was 11.

Remarkably, there has hardly been a voice raised to question her account in Austria, where a population horrified by the cellar revelations has puzzled foreigners by showing a determination not to indulge in national soul-searching. Part of the reason may be the instinct of ordinary Austrians to mind their own business, even when it comes to the misdeeds of neighbours.

Mrs Fritzl's sister has no doubts that she is a victim. "Apart from her family she has no one to support her," Mrs Renner said.

"She is physically and mentally at the end of her tether. The only thing that keeps her going is the thought of the children. When they are not around she can't help crying a lot of the time, but when she's with them she seems to find the strength to put on a brave face."

There appears to have been a rapprochement of sorts between Elisabeth and her mother. The daughter, now 43 and grey-haired, takes her children to see their grandmother once a week, both the "downstairs" children about whom Mrs Fritzl didn't know, and the "upstairs" children whom her husband had presented her with to raise - and who still regard her as their mother.

Elisabeth has never spoken publicly about her new life, but according to Mrs Renner the meetings with Mrs Fritzl are not always easy.

Mrs Renner said: "There is a lot of friction between her and Elisabeth. Of course they talk to each other about general things and spend time together.

"But neither of them can bring themselves to talk about that which is really at the centre of everything. The only things that Rosemary knows about her daughter's ordeal are the things she reads in the newspapers. She didn't realise Elisabeth was kept on a dog lead for the first few months until I told her.

"It was only when she asked Elisabeth about it that she said yes it's true. Elisabeth can talk about many things but she can't bring herself to talk to her family about her ordeal."

The children are said to have healed to some extent in the last year, especially six-year-old Felix, the youngest. It has been more difficult for the older "downstairs" children to adapt – they spend most of their days watching television – and reports have suggested it has been hard for the "upstairs" children to bond with their siblings in the reunited family, although they have tried.

In the next week the world is unlikely to hear an answer to the question of why they were forced to lead such a bizarre life.

The trial will only last five days, running to a strict timetable, and it will end on Friday with an inevitable guilty verdict and a sentence which will leave the pot-bellied pensioner in jail for the rest of his life for murder, rape, sequestration, incest, grievous assault, and slavery.

Fritzl's defence lawyer Rudolf Mayer has said that the accused will only contest the murder charge - which relates to the death of his new-born twin son in 1996, after he failed to call medical help. Mr Mayer has said that is client, now 74, expects to die in prison, but insists that he is not a sex monster and that he loved his daughter in his own way.

The charge sheet puts it differently. It reads: "He shut her away in the cellar and made her totally dependant on him, forcing her into sexual acts and treating her as if he she was his own property."

Jurors will be told that he was sane. A psychiatric assessment indicated that he was fit for trial and aware of his actions despite a severe personality disorder.

In prison awaiting trial he told doctors that he felt guilty about what he had done, especially when he lay awake at night.

The press and public have been banned from most of the hearing except its very beginning and end. Judges have ruled that because the evidence is so harrowing, jurors must spend no longer than two hours a day listening to it. Elisabeth will not attend but hours of her pre-recorded video evidence will be heard.

By now many of Fritzl's countrymen take little effort to hide their weariness with the extraordinary case, which came to light less than two years after teenager Natascha Kampusch escaped from a basement where she had been held for eight years.

After the verdict is passed there is little prospect of an inquiry, despite a trail of embarrassing mistakes by officials and police officers who for years missed the abuse.

"It is the most horrible case in Austrian criminal history," said district governor Hans-Heinz Lenze last week. "But people here want to forget it now. They have had enough of hearing about it. In Amstetten it is 'business as usual'."

Regina Schoeller, who worked in a shop opposite the concrete housing block where Fritzl lived, was blunter. "I am tired of hearing about evil Amstetten and evil Austria," she said. "It could have happened anywhere."