John Dally died on 25 August 1994. He was 35, gay, and had found out that he was HIV positive in 1986. In the summer months before his death, as his life tapered away, his family and friends collected at his bedside in Islington, north London, to administer watermelon and diamorphine. This was a grim, often abrasive reunion, a last gathering of the clan before John's final departure. Now his sister Emma Dally, a novelist and publisher, has written a memoir of John's illness - which indirectly describes the suicide of their eldest brother several years before - called Dying Twice.

The death memoir was the cultural and publishing sensation of the past decade. Think of the voices from the brink: Oscar Moore, Jean Dominique-Bauby, Ruth Picardie and John Diamond. Dally's book, like John Bayley's account of the decline of his late wife Iris Murdoch, is a commentary from the sidelines, and perhaps critics - and this genre has plenty of those - will say that as a mere sibling, her experience is somehow less valid than his. Dally does not care. Like her book, she is direct, unapologetic and disconcerting. 'I thought it was essential to write about death in a very down-to-earth way... I get so sick of all the euphemisms.'

She turns 46 tomorrow, and has already been to the funerals of two of her brothers. She does not see why she should be delicate about this, why she should apologise for writing about it. 'When I sold the book, I got a bigger advance for it than I had for my novels,' she says, with a brusque satisfaction that is, inevitably, shocking. 'My editor says it's extraordinary how death has become a kind of trendy subject. A few years ago, no one would have touched it, but now everyone's interested.'

How does that make her feel?

'That I'm on the cutting edge,' she fires back. 'Commercial trends reflect cultural changes, don't they, reflect what people are thinking. I've no problem with that.'

At the beginning, there were six Dally children: Simon, Mark, Emma, Jane, John and Adam. Their father, Peter, was a psychiatrist; their mother, Ann, a doctor and medical journalist. The children were raised in London and at the family bolthole, a house in Sussex called Wiblings. Emma was five when John was born. Two years later, Adam came along. 'I still despised dolls because I did not need them,' she writes. 'I had real babies to dress and bathe and play with. I loved to give them their bottles, swing them casually on my hip the way I had seen the housekeeper and my mother do it, and push the pram down the road as though I was a mother myself... Babies dominated the household, and I loved them.'

Emma, the little mother, did well at school, went to Oxford and then followed her eldest brother Simon into publishing. John's dyslexia foxed the system, and he left school without A-levels before setting up a courier company with Adam. At 24 he was driving a silver Mercedes, sending back his food at Le Gavroche, and being hailed as the quintessential Eighties entrepreneur in the press. When he told his family he was gay, it was no big deal, but since this was the early Eighties, and the papers were beginning to report on a new disease that afflicted gay men, Emma was relieved when he settled down with the fashion designer Jacques Azagury.

Then Simon, their oldest brother, became ill. In 1988, he was diagnosed with manic depression. He stole some antiquarian books from a private collection and was arrested. A few weeks before the court case, he joined the rest of the family at Wiblings where, on Easter Sunday, he lay down on the front lawn and shot himself. His body was found by Emma and her husband, the food writer Richard Ehrlich, when they and their daughters returned from a walk. The date and place were not random, fuddled choices, says Emma. Like her father, she thinks Simon's death, there in the garden, 'was in a way a criticism of the family for not helping him. Family meant so much to him, but he hadn't been helped. That was typical Simon.'

After Simon's death, during the grieving, there were arguments over money. John and Adam's courier business was in trouble, and there was a legal wrangle over Simon's estate; it also transpired that they had persuaded their mother to put up Wiblings as collateral. An 'operatic' row was brewing. By the time the news spread about John's HIV status, the siblings were split down the middle, and Mark and Jane were not speaking to their younger brothers. 'Someone asked me whether my family was particularly dysfunctional,' says Emma, 'and I don't think it is in the slightest. It's just very large, full of people who say what they think.'

This is not a book about family grief: the Dally parents, now divorced and both based in the country, are in many ways striking in their absence. It's a story about brothers and sisters and a handful of friends behaving well and very badly in awful circumstances (falling out for imperceptible or enormous reasons, and scrabbling to make it up in time) while John, charming, naive and utterly impractical, sinks deeper into the disease.

In her book, Emma recounts all of this without bothering to clamber on to the higher ground. If some of the participants, including John, sometimes come across as careless, greedy or insensitive, she comes across as priggish and stubborn. In that way, it's a strangely dispassionate narrative, and more authoritative for it. 'I just knew that if I was going to do it right, I had to be frank about it all. In a sense I had this set view of how people should behave around someone who's dying which was just as bad as anything else. There isn't actually a set plan of how to react. Some people think they grieve more than others - but people grieve in very different ways.'

Though John's circle will never be the same after his death, most of his friends have maintained links with the Dallys. The exception is Jacques Azagury. Despite the fact that their relationship had ended by then, Jacques spent the last months with John and the family, but severed links after the funeral and another row concerning finances. While Emma was putting the book together, he rebuffed her attempts to communicate with him. 'I don't think there's any unresolved business,' says Emma briskly. 'No, and I don't feel that he wants to be friends. He didn't feel the warmth for me that I felt for him, that was obvious in the end, and that was very sad. But there's nothing to make up. We couldn't be friends. It was a horrible way for things to end.'

So who has Emma written the book for? Partly for people with terminal diseases, especially gay Aids sufferers ('This Section 28 thing drives me bonkers. All these church people talking about homosexuals as though they're a different breed. It's unbelievable') and their families. Partly for herself ('Writ ing helps you work out what you think and feel about things. Until I did it, things were in a great big muddle'). But mostly for John ('Simon had two nice obituaries, and Johnny didn't').

The book was her mother's idea. If the suggestion hadn't been made, Emma is not sure whether she would have thought of it. She's not expecting an easy ride, but she doesn't give the impression of being particularly anxious about the reception, either from critics or family members. Writing about death, she says, one is guaranteed to offend. 'People get very upset about it. Who are they to say whether you can write something or not? But that's the wonderful thing about taboos. They get people worked up, so they lose sight of everything.'

She talks combatively, as if looking forward to a scrap. 'There is this funny thing when people die, about who owns the information. And I have to keep reminding myself that I'm entitled to have my perception of this, and to write about it. And even if people get angry and say, "It wasn't like that" - well, it wasn't like that for them, but this is my view." If you grow up as one of six siblings, even if you end up losing two of them, you learn how to fight your corner.'

• Dying Twice: A Sister's Tale is published by Little, Brown at £16.99