Meeting Piers and Hilary du Pre is an unnerving experience. It's like opening the front door to the Plymouth Brethren. You know from the moment you see their fierce bright eyes that there is no argument, no logic that can dent their belief in their own righteousness, a righteousness that comes from the certainty that they are doing the great one's work. In the case of the du Pres, it is their sister Jackie rather than God who has given them "the courage" to tell "the truth of Jackie". Yet this truth is so at odds with the authorised version that it has the musical world raising its voice in a cacophony of condemnation, including du Pre's former husband, Daniel Barenboim, who is reported as saying: "Couldn't they have waited until I'm dead?" Jacqueline du Pre, A Genius In The Family, [is, to say the least, unsettling. Both for the personal revelations about the cellist herself, who comes across as a sexually voracious, stunningly immature, demanding monster, and the family that spawned her.

From the earliest days everyone else's needs were subjugated to Jackie's "genius", with both siblings serving as acolytes to their sister. Piers rebelled and one day just downed his clarinet, never to pick it up again. He found his salvation as an airline pilot - a perfect metaphor for escape.

Hilary, an equally precocious musician, was less lucky: "And how is your wonderful sister?" dogged her own career. She changed instruments (piano, violin, flute) but the relationship never changed. Hilary's role was that of accompanist. She was put into a technical strait-jacket that emotional Jackie was allowed to ignore. As a result she became increasingly incapable of performing. At 19 she escaped through marriage. But the pattern continued. Whatever Jackie wanted, Jackie got, including, ultimately, Hilary's husband Christopher (Kiffer) Finzi. Brother and sister are mystified by the furore their book has caused. Piers speaks slowly, like a benign primary-school teacher, using words like "gosh" and "crumbs". Hilary, wide-eyed and neat, could have walked straight out of The Sound Of Music. All they wanted to do in writing the book, she says in her clipped 1950s voice, was to set the record straight. When I suggest another motive might have been revenge - in the circumstances quite reasonable - they are equally mystified. How might I imagine such a thing? "Do we look damaged? Do we look unhappy? Jackie was a stunning person". Their faces beam.

"Jackie was set up as a star, as an icon, as a saint," explains Hilary. "And that's a terrible way to have to live. Jackie was a human being. Her life was very very difficult for her because she was the possessor of the most enormous talent. Huge genius. And that's a very very difficult thing to bear."

For whom? "Why for Jackie of course." With both parents now dead, Piers and Hilary were the only people who could tell the story from the inside. Yet a would-be biographer had told them their roomful of memorabilia "might be worth a chapter". They were horrified. What to do was the most agonising decision either has ever made. "Either we shut our mouths and burnt it so no one would ever know the truth or we had the guts to write it down."

Those who berate them, they reason, simply can't have read the book. "People who have read it have been enormously sympathetic and understanding. It's the ones who haven't who have done the attacking."

Both Piers and Hilary are adamant that, whether seen from their own perspectives or Jackie's, there are no lessons to be learnt from Jackie's life. "Who can point a finger at a parent and say you have made a mistake with that girl? Although we love her playing, you made a mistake."

It is not the first time that the du Pre family have fallen foul of the musical world. It happened 22 years ago at the onset of the MS that quickly crippled Jacqueline du Pre and less rapidly (14 years) killed her. Then it was their mother, Iris Greep, a concert pianist, who was seen as the chief culprit.

"When Jackie became ill, the world that had loved Jackie and loved her playing so much, suddenly didn't have it any more. And it made a lot of people very angry." They wanted someone to blame, says Hilary, and chose the family. " `No wonder she's got MS. Look at her background. She didn't have any friends. She didn't go to school. No wonder.' No one turned round and said, my goodness what we've had is absolutely wonderful. Thank you Iris, thank you Jackie." That was long before the extraordinary revelation that following a breakdown in du Pre's marriage with Barenboim, she lived at her sister's farm in Hampshire, and slept with her brother-in-law. Hilary had no inkling of what Jackie had in mind until she crept into the marital bed, soon after her arrival from New York in a desperate state. "Kiffer was in the middle, lying very still," Hilary writes. "She was doing her best to rouse him. I, pretending to stir in my sleep, rolled over and put my hand on Kiffer . . . Although I knew what Jackie needed and wanted, I was incredulous with disbelief. How could she? How could she? How could she even think of taking my Kif?"

The incredulity of that moment appears short lived. After due discussion husband and wife agreed it was the only thing to do if Jackie's genius was to be saved. The arrangement lasted for more than a year, until Kiffer was given a book by R.D. Laing, and it finally dawned on him that "this is what Jackie needs". Although this was 1972, Piers, Hilary and Kiffer claim never to have known about the existence of psychoanalysis before then. Rather naive, surely?

"Perhaps we were," replies Piers. I ask Hilary if she and Kiffer had, as was suggested by Jackie herself to a friend, an "open" marriage in which such infidelities were an accepted part. She side-steps, and with great pride tells me that they have just celebrated their 36th wedding anniversary. In retrospect does she feel that Jackie's sexual relationship with Kiffer was wise?

"I think it was the one thing that enabled her to keep going at the time. Jackie came to us for help. We did all we could to restore her, to get her back on her feet. So she could go back to her playing which she loved, but which at that moment she didn't love. When we eventually found a psychoanalyst she was very much better than when she'd come. I think that when she did come she was so low that she would not have benefited from the psychoanalysis."

The du Pres may finally have their revenge, at least financially. Filming of the movie, starring Emily Watson ("she has Grade One cello you know") starts on December 4th. It is unlikely that, without the added frisson of the menage a trois, it would have happened. But baring all was necessary, says Piers, simply because it was the truth. And Jackie deserved the truth.

Piers knew nothing of the bizarre domestic arrangements at the time - he was away learning to fly. When he did find out, during the process of writing the book, he was surprised but not shocked.

"It was like finding the one missing piece of jigsaw. It was absolutely brilliant. Lovely." He beams. "Without it the picture would be incomplete. Someone somewhere was bound to know and to have that thrown in as a googly five years down the line, 10 years down the line, tomorrow, would have shown our book to have been stupid. How on earth could you then say ours is the proper story? It had to be included. I'm sure Jackie would not have minded."

A Genius in The Family - An Intimate Memoir of Jacequeline du Pre is published by Chatto and Windus. Price £16.99