TAMASIN Day-Lewis is giving vent to her frustrations. "It's a war of attrition!"she says, her eyes fixing on mine with a blow-torch gaze.

For three years, she has been trying to make her first feature film. She has the rights to The Private Wound, the last novel by her father Cecil Day-Lewis, and a script - but she is £1 million short of her budget. "Ultimately," she explains, "it's all down to having the right cast."

Well, I suggest, why don't you ask your brother Daniel to star in it? Eight years ago, the Oscar-winning actor flunked out of a stage production of Hamlet because he was said to be haunted by his own father's ghost - so the lead role in his father's romantic thriller, based on a pre-war affair, might even prove therapeutic . . . "I wish he would take a part; I would be flooded with money," says Tamasin. "But he won't. I could kill him."

Fratricide is unlikely: Daniel Day-Lewis's elder sister understands his position too well for that. "He is ruthless about the parts he takes. He has to be; people want to topple him. He only does something if he has an absolute will to do it. Don't forget: he turned down Schindler's List and The English Patient."

Maddening as it is to spend another summer looking for finance, she is also relieved not to be making the transition from documentary-making under his shadow. "However well we get on, there would be problems. And I don't want it said that the film was made only because my brother was in it."

Before our meeting - set up to discuss her new cookery book - I had been nervous about broaching the subject of Daniel with the sister who has striven so doggedly to make her own name. Tamasin had no such worries: "I'll tell you anything except his inside leg measurement," she'd said gamely on the phone.

Ever since she first strode across my path at Cambridge University - in Manolo Blahnik high heels and wolfskin coat (the gift of an older lover) - she has been anxious to achieve something in her own right. Even 20-odd years ago, she did not fall back on her connections: her father was poet laureate and her grandfather was Sir Michael Balcon, founder of the British film industry. And now, despite a few passing regrets about her film, she is no keener to milk the link with her brother.

In West of Ireland Summers - a book of her own recipes, laced with an evocative account of the holidays of their youth - she mentions him only in passing: boiling shrimps with her on the beach or selling mackerel on the quay for 1p each. But she never lets his angular presence dominate. The book itself is weighted with the kind of calorie-laden concoctions that only one as stick thin as Tamasin can cook without fear.

Cooking, she explains, is her way of showing affection. Her passion for it began when she was a schoolgirl, developed at university as a reaction against students who saw food only as "fuel" and grew to an "obsession". "I wanted to please everyone," she explains.

I had last seen Tamasin four years ago at the mill house in Somerset where she lives with her husband, John, a BBC executive. At the time, a large station wagon belonging to "Wicked Uncle Dan", as their three children call him, was parked in her garage and she was leafing through estate agents' particulars to find him a house in Ireland, the land of their father's birth.

She was enjoying his fame - talking readily of how she had sat him on the sofa and fed him buckets of icecream when he came to stay - but was wary of discussing his delicate psyche.

Wherever she goes, the people she meets are always trying to get an inside track on Daniel. Even Ireland, where she has a cottage in County Mayo, provides no relief. "The Irish are worse; they come to the door," she says. She sends the nosey packing.

Wary of being classed as such, I ask her about her work as a documentary producer. Seven years ago, she was forced to leave the BBC because of a rule against husbands and wives working together.

Sending out countless proposals and coping with the inevitable rejections has been tough. Although Tamasin's various series on adultery, gifted children and letters from Second World War soldiers have been well received, she lives in a constant state of insecurity.

Does she not envy her younger brother's success? "I would love to have people begging me to fill slots, just as he has piles of scripts waiting for him. And it would be useful to have more money," she admits.

"But I don't envy him his fame - not when I see what has happened to him. He has no privacy. Everyone feels they own a bit of him, that they have a right to know."

In the face of gossip, she says, Daniel maintains a dignified silence. "It's the only way," she believes. "I get crosser than he does and call up newspaper editors. I can't bear the way he is vilified."

The tale of how he dumped the French actress Isabelle Adjani - then seven months pregnant - by fax is "pure fabrication," she says.

Nor does she have a particularly warm recollection of Adjani. "When she came to stay at my home in Somerset, she never took her sunglasses off. She probably wore them in bed." Tamasin's nephew, Gabriel, is now two years old, and she has never set eyes on him.

Last November, when her brother finally married, another story illustrating his "callous" behaviour did the rounds. Deya Pichardo, the personal trainer who lived with him in New York, claimed that she had only learnt of the marriage from Adjani. This is bosh, says Tamasin.

"They had broken up months before; he was kind enough to let Deya stay on in the flat."

Sadly, the persistent image of Dan as the heartless Lothario has caused a rift within the family.

In April, there was a fracas over the unveiling of a statue to Sir Michael Balcon. Jonathan Balcon, their mother's brother, wrote to the organisers saying that it would be best if Daniel stayed away because Sir Michael would have been "horrified by his sexual mores".

"It was completely horrific and not what my grandfather would have wanted," says Tamasin. "He would have thought it was extraordinarily vulgar."

Tamasin's own relationship with Daniel is so close that it amounts, she says, to "a conspiracy". They work similarly - by total immersion. He had rung her the night before while filming the lead role in The Boxer and had spoken in the Northern Irish accent he is using for the part; this did not strike her as at all odd.

For the documentary she is currently cutting - a series on musical prodigies at Chetham's school in Manchester - she abandoned her family to live there all winter. "It's the only way to do it, from the inside," she explains.

She never speaks less than emphatically, never works less than flat out and, like her brother, produces work of rare intensity. Daniel, she says, "burns a hole through the screen; it's not acting, it comes from within. I try to film people who have that quality. I am fascinated by those who have the will to achieve - their courage, conviction and insecurity."

Where does it come from? "Our parents were ferociously anti-competitive; perhaps that makes you ambitious. We also had examples of fantastic achievement, in my father and grandfather, which made us feel that you did something because you loved it and you gave 115 per cent."

Her battling quality was probably also a response to nursery life in the Day-Lewises' Greenwich house. It was an upstairs, downstairs world, where the children saw grown-ups as "the enemy". Their mother, the actress Jill Balcon, has since repined that she was so absorbed in their father (and acting) that perhaps she didn't pay enough attention to the children.

Contact between the two worlds was limited to Sunday lunch and bedtime stories. The rest of the time, Tamasin remembers, "my father was downstairs in his study, reading or writing. I was upstairs reading, writing, drawing or playing acting games with Dan.

"I remember him leaping from the nursery window sill as Yuri Gagarin, the first man in space."

Their father's illnesses - he had several heart attacks before he died of cancer - accentuated that early independence. "When I was 11 or 12, he began to be carted off to hospital. I always thought he was going to die," she says. "I was the daughter he had always longed for, and the elder, so I was able to talk to him as a teenager. Dan didn't so much. After his death, we became self-sufficient - but we had each other."

Tamasin was 18 when he died; Daniel 15. Before going up to university, she left home and worked as a model for two years, living in a flat near Marble Arch, where she would marinate venison for 25 in the bath. Daniel went to Bristol Old Vic drama school and, when Tamasin went there to work for the BBC in 1979, they shared a flat.

These days, they talk frequently, announcing themselves on the telephone with coded signature tunes, sometimes discussing work, sometimes their late father's idiosyncrasies. In times of trouble, Daniel always turns to her.

"He knows I won't ask questions," she says. Instead, she encourages his playful side: "We sing and dance, fall about in hysterics or watch Clint Eastwood movies. My children think we are demented."

She is delighted with her new sister-in-law, Rebecca, the daughter of Arthur Miller, and was tickled when the playwright remarked on their striking resemblance. The wedding, in a Vermont village, where Miller's oldest friend is vicar, provided "two or three of the happiest days" of Tamasin's life.

"Rebecca is the best thing that could have happened to Dan. Having been brought up in the shadow of a famous father, she knows what it is like. And she has a life of her own as a film maker; she won't get swept away."