Piers Dudgeon's contentious, aggressive but haunting study Captivated offers an account of what he finds to be the malign influence on the Du Maurier and Llewelyn Davies families of JM Barrie and the Peter Pan story. Jane Dunn  who rejects Dudgeon's findings  comes at the Du Maurier story from a different angle, but leaves us in no doubt that there was something seriously odd about them.

Her book belongs to the growing genre of what might be called Sisterly Feelings; Paula Byrne's excellent recent The Real Jane Austen and Dunn's own A Very Close Conspiracy: Vanessa Bell and Virginia Woolf are notable examples, though perhaps one of the greatest is Daphne du Maurier's own The Infernal World of Branwell Brontë, in which the brother's tortured life vividly illuminates those of his three remarkable sisters. Unlike the Rev Brontë's girls, Gerald du Maurier's three daughters were not equally touched by genius, which sets a problem for biographer, compounded by the refusal of the surviving partner of the youngest, Jeanne, to allow the author access to her archive. Both she and the eldest sister, Angela, led fascinating lives, but Daphne was by far the most original of them, the most productive and, needless to say, the one best able to write about herself, which gives her an unfair advantage in terms of the reader's interest.

She was also the favourite of her father, an actor whose mercurial, alarming presence dominates the first half of the book. Here Dunn has been upstaged by Daphne, whose extraordinarily candid and superbly written biography Gerald, which appeared shortly after his death in 1934, unsparingly depicts him as the epitome of the popular view of actors  feckless, leaping from one emotion to another, one persona to another, charming, unreliable, bitchy, childish. She had already drawn on him in The Progress of Julius, the novel she wrote a year earlier, when Gerald was still alive, presenting a startled world with a portrait of a father-daughter relationship that was more like a love affair; she expanded this portrait 15 years later in The Parasites.

Dunn makes it clear that though Gerald made sure the whole household revolved around him, Daphne was able to hold her own. Her relationship with her mother, who idolised Gerald and deeply resented his fascination with their daughter, was cool, which, Daphne said, turned her into a "dreamer of dreams", suspicious of all adults: "You could never be quite sure of any of them, even relations." Muriel du Maurier evidently felt unthreatened by Jeanne, but was fairly indifferent to Angela, the least pretty, who felt unloved by either parent. All three daughters escaped into a fantasy world of their own, inventing male personas for themselves. "Sisters?" wrote Noël Welch, Jeanne's girlfriend. "They should have been brothers. They would have made splendid boys." Daphne was Eric Avon, a dashing captain of games, and derring-doer; Jeanne became David Dampier. Sometimes these later egos slipped into the real world: the actor Roland Pertwee, staying with the Du Mauriers, was surprised to find Jeanne in his room, having folded his trousers and put toothpaste on his brush: "I'm Dampier and I'm your fag. Shout if you want anything else."

The creation of this alternative reality is characteristic, not only of the girls, but of the Du Mauriers in general: "The Du Maurier character was volatile and charming," writes Dunn, "inflated with fantasy and pretence; characteristically, they hung their lives on a dream and found little solace in real life when the romance had gone." George du Maurier, Gerald's father, the famous Punch illustrator and author of the great 19th-century bestseller Trilby, wrote, in his debut novel Peter Ibbetson, a charter for the imaginative life, in which the hero formulates the notion of "dreaming true", a method by which everything the dreamer desires comes to him.

Pretty well everything Daphne desired came to her: a series of bestselling novels, a nobly handsome war hero husband and, above all, a house, Menabilly in Cornwall, which both obsessed and liberated her. But her relationship with reality was distant. The glory of Menabilly (a house no one could see the point of but her) was that living there, on that wild coast, was like living in a novel. Jeanne, too, dreamed true: a more modest dream of living as a painter, with a loyal partner at her side. Only Angela's dreams proved unreliable: she dreamed of a boyfriend and a husband, but instead she had a series of affairs with some remarkable women, the grandes horizontales sapphiques of her day. Dunn is excellent on the lesbian 1920s and 30s in London, with delicious detail  Lena Ramsden, for example, insisted that the perfect present for young women she was courting was a trouser press.

Angela wrote, rather badly, about her gay inclinations, and her novels in general are clumsily written and overwrought; she found her form as a memoirist. Jeanne wrote nothing, but the few letters Dunn has had access to express the ardour of her feelings for various women. Daphne never wrote about her sexual and emotional experiences with women, but in some remarkable letters written in her 40s to a woman she had a deep crush on  her American publisher's wife  she described how she did not see herself as lesbian. When she had loved women, as with her long first affair with the headmistress of her finishing school in Paris, it was not as herself, but as Eric Avon: "At 18 this half-breed fell in love, as a boy would do, with someone quite 12 years older than himself who was French and had all the understanding in the world and he loved her in every conceivable way up to the age of 23 or so. And in so doing he learned almost all there is to know about that complex thing, a woman's heart." And then, she continues, "the boy realised he had to grow up and not be a boy any longer, so he turned into a girl, and not an unattractive girl at that, and the boy was locked in a box and put away for ever." Except at Menabilly, where she "sometimes let the phantom who was neither girl nor boy but disembodied spirit dance in the evening when there was no one to see".

In fact, the boy-in-the-box, as she referred to him, had a few more outings in him, including a deep passionate tendresse for the actor Gertrude Lawrence, not coincidentally one of Gerald du Maurier's last co-stars (and lovers). But, as Daphne freely admitted, her relationship with her children and her war-shattered husband were cool; her inner life was the only one that really mattered. There is something faintly obscene about her detachment, as Dunn notes: "Fortified by the champagne and roses of life at Langley's End, Daphne could still watch a formation of 20 German bombers on their way to bomb Luton  and see the beauty of them rather than the deadly menace they embodied." Her passion for Menabilly is also bizarre, an almost spooky houseophilia. Her daughter Flavia once watched her kiss the stone wall: "When she turned, her slightly flushed face had a look close to ecstasy." She felt that "the house is in league with me against the world".

As Daphne dominated the lives of her sisters, she dominates Dunn's pages. Her sisters' lives, especially during the war  Jeanne nobly laboured on her vegetable patch, which almost destroyed her health, while Angela cleaned the cowsheds  cast a very interesting sidelight on what responsible middle- and upper-class women did for the war effort; indeed, they might each merit a book of their own. It is not clear that Dunn's sweeping contention that "in biography, families are the soil out of which character grows, and there is no richer compost than the relationship of sisters" is entirely proven. The problems of contrapuntal writing particular to group biography are not really solved; a paragraph about one sister just follows one about another. And the book is woefully underedited: Noël Coward, Signor Staccato himself, is described as "drawling", something he was constitutionally incapable of; we have Tormanova for Toumanova, Schofield for Scofield, non-representative art for non-representational, Lesley Hutchinson for Leslie, turning the most rabidly masculine of men into a girl.

But Daphne (and Gerald, it has to be said) constantly shine through. Towards the end of her life she became interested in the psychoanalyst Alfred Adler and his theory of power complexes. "It seems we all underlying want power over our fellow human beings, and our life-plan was settled at the age of five  mine was to be left alone, and not go down to the drawing-room. So it still holds good! But it is lonely, being alone."

Jane Dunn's biography of Daphne du Maurier (aka "Bing") and her sisters, Angela ("Piffy") and Jeanne ("Bird"), arrived on my desk trailing mystery and excitement. Its campy subtitle, after all, refers to "hidden" lives. Hidden. For biographers and their publishers, this is such a tempting word, hinting discreetly at secrets, lies and  in this instance  sibling rivalry of a particularly sticky, stabby kind. The momentarily thrilling thought occurred that Margaret Forster's 1993 biography of Daphne (it was Forster who revealed the novelist's complicated sexuality, and her obsessions both with Ellen Doubleday, the wife of her American publishers, and Gertrude Lawrence, the actress) might finally have a rival.

But, no. She is safe for now. Dunn has nothing much that is new to say about Daphne. This version of the writer is just as introverted and as selfish as the last, and the broad narrative of her life will be familiar to fans: the obsession with a Cornish house called Menabilly; the mostly unhappy marriage to the soldier and war hero, Tommy "Boy" Browning; the neglect of her daughters; the incredible success of her strange and seductive novels. Given her extraordinary fame  Neville Chamberlain was reading Rebecca when he flew to Munich to meet Hitler in 1939  it goes without saying that her sisters lived in her shadow. Angela, the eldest of the three, wrote some bad novels and some slightly better memoirs. Jeanne, the youngest, was a second-rate painter on the fringes of the St Ives school. Both were lesbians. Were the three of them deadly enemies? No. Daphne failed to show up to Jeanne's first exhibition, and was unable to bring herself to praise Angela's creaky melodramas (white lies, you gather, were not exactly Daphne's thing). But her siblings seem not to have cared overmuch. Jeanne ploughed her own furrow  quite literally during the war, when she took up market gardening. Poor old Angela, who called her autobiography It's Only the Sister, found Daphne's novels utterly thrilling, and told her so. They died  Angela was the last to go, in 2002  as friends.

The fact that the sisters were not rivals in adulthood is, perhaps, Dunn's most surprising discovery, given that they grew up in a household where "histrionics were a way of life". Home was dominated by their needy and mercurial father, Gerald, the actor-manager who had made his name playing both Captain Hook and Mr Darling in JM Barrie's Peter Pan, a play with which all three sisters were weirdly obsessed as children. Gerald was reluctant to grow up, and he wanted to keep his girls babyish, too. "It isn't fair!" he cried, when Daphne, his favourite, announced that she was to be married. His daughters' lives were threaded with glamour: Valentino and Gary Cooper came for Sunday lunch; Daphne screen-tested with Ivor Novello; Angela was cast as Wendy in a revival of Peter Pan (a career that ended in ignominy when, in a flying scene, she crash-landed in the orchestra pit). But they were impossibly stunted, too: ignorant about sex, easily distracted and prone to swoony crushes (these were mostly on women, in spite of all that daddy had told them about lesbians, of whom he had a horror). Jeanne decided that she was really a boy early on. A male guest at the Du Mauriers' palatial home once woke to find her putting toothpaste on his toothbrush. "I'm Dampier, your fag," she announced, exiting the room. "Shout if you want anything else."

Life only really began in earnest after Gerald's death from cancer in 1934. Deep in grief, Daphne, his favourite, turned to her writing; Jamaica Inn was published in 1936, when she was 29, and the singular Rebecca came hard on its heels two years later. Jeanne, released at last from her father's loathing of modernism, began to experiment with colour and paint. Angela, meanwhile, headed to Italy to see her cross-dressing friend, Naomi "Micky" Jacob, a pal of Radclyffe Hall and a writer of novels as bad as her own. Back in London, with no daddy to keep her home, she was happily absorbed into Hampstead lesbian circles, a world of febrile love triangles and trouser presses (a favourite gift, apparently), until she fell in love with Winston Churchill's aunt, Olive Guthrie. Thirty-two years her senior, Olive lived at Torosay Castle, on Mull, with a Pekinese and a parrot. The parrot had previously shared its quarters with the butler, and every time it heard the ring of a bell, it squawked: "Let the old bitch wait!"

Jeanne's long-term partner, the poet Noel Welch, refused to co-operate with Dunn on this book; possibly because of this, she struggles to bring her alive on the page. She does a little better with Angela, indulgently analysing her excruciating poems, doggedly recording her every pash. But still, it's hard to see why either of these women are of any real interest, except in relation to their more famous sister  and perhaps, deep down, Dunn was aware of this, for her book feels strained at times. It's maddeningly repetitive (the text seems to have been edited hardly at all), and her style is painfully convoluted and overwrought. Cliches abound  "Daphne drew men like moths to her flame"  not to mention strange inelegances (some women, having slept with Vita Sackville-West, were "loath to return to the marital bed and the usual wham, bam, thank you ma'am").

Mostly, though, its longueurs are a simple problem of organisation. Her decision to deal with all three women at once, and chronologically, rather than in separate sections, has dealt her narrative a fatal blow. Stop-start, stop-start: no sooner has she got going on one sister, than she must turn to another. She could have been more creative, more radical, and much less reverential. All three of these lives turn on one or two significant moments. She might have begun with these, and worked back, ruthlessly cutting as she went. I can't speak for Piffy and Bird, but I think that Bing, that great mistress of narrative pace, would have rolled her eyes at this book, and set about its more laboured passages with a sharp, red pencil.