John Lennon: What really happened in his childhood We thought we knew every detail of John Lennon's life. But his half-sister kept quiet about what really happened in the Beatle's childhood - until now. John Walsh reports

Each unhappy family, as Tolstoy remarked, is unhappy in its own way - but the great Leo could never have anticipated how a family's unhappiness could be worsened by the accretion of half-truths and Chinese whispers in the celebrity media and publishing circuit.

Julia Baird knows this all too well. As the half-sister of John Lennon, she's had to monitor a blizzard of inaccuracies about her beloved sibling. "Our hidden histories have been hung up across the giant screen of the sky," she writes, "inviting inspection and criticism from all and sundry, and dissection from Beatles experts and John experts." Now, though, she has made a valiant stab at setting things to rights.

It's a tragic story, and at its centre is Julia's and John's mother, also called Julia. She was one of the five Stanley sisters - Mimi, Betty, Anne, Julia and Harriet, all born in the shadow of Liverpool's Anglican Cathedral. Julia Stanley - red-haired, exuberant, musical and headstrong - was only 14 when she began seeing a hotel bellboy, Alfred Lennon, to her parents' chagrin. Alf became a ship's steward and spent long periods at sea, but their romance survived his absences. They were married in a register's office in 1938 with no family members present; Alf put to sea the following day. When war broke out, the Liverpool shipyards were bombed but the family (now living in Penny Lane) survived. Alf, now a merchant seaman, came home long enough to make Julia pregnant, then decamped across the Atlantic. The baby was named John Winston Lennon. With the child's father mostly out of the picture, Julia and John moved in with her disapproving father. Julia became pregnant by a passing Welsh soldier and was persuaded to give up the baby girl for adoption. Then, while waitressing, she met "Bobby" Dykins, a demonstrator of invisible mending, and they fell in love.

What followed has been the stuff of much confusion. As several Lennon biographies will tell you, the five-year-old John went, by arrangement, to live with his aunt Mimi in a house nearby called Mendips, while his mother started another family with Bobby. Unable to marry, because the chronically absent Alf was still alive, they had two children, Julia and Jackie, while John would pay the occasional visit. That's the representation of life that Baird is anxious to overturn in Imagine This: Growing Up with my Brother John Lennon.

She has been a tenacious guardian of his flame since 1985. "Only five years after he died, there was a BBC 'celebration' of John's life that I watched and it was so badly wrong," she says. "I felt I had to do something, so I put together a handwritten, limited-edition copy, using all the family photographs. I got it properly published in 1988. But the story is still escalating. I still hear and read things." Such as? "That my mother gave John away. That she went to live with a man who had two children from a previous relationship - [her eyes blaze with indignation] as if my sister and I weren't born to our mother at all!" The truth, it seems, involves the grotesque, condemnatory figure of Aunt Mimi, who waged a bitter war with her own sister for possession of the little boy, claiming that Julia and her new man were disgraceful public sinners; their house an unfit arena in which to bring up a child. She effectively kidnapped John and barred the door against poor, distraught Julia when she called to see her son.

What brings a tremble to Baird's voice are the revelations she unearthed in researching the past. She discovered, for instance, the existence of her half-sister, the baby sent away for adoption. And through a fog of mutterings and hints by her Aunt Georgina (known as Nanny), Baird gradually revealed that Mimi, the sainted, hell-and-damnation moralist, had for years been sleeping with her lodger (she in her fifties, he in his twenties). Baird contacted the ex-lover, whereupon he confirmed the affair, and the fact that Mimi, despite being married, years before, had been a virgin when they got together.

This opens a whole can of psychological worms about the reasons for Mimi's appropriation of John. "People come to terms over relationships, don't they?" said Baird. "Mimi and her husband obviously came to that agreement [ie not to have children] before their wedding day. But I've come to the conclusion that her taking John away was an act of opportunism."

Her book is an act of worship to a brother she clearly adored, but is also a tenderly evoked memoir of a Liverpool childhood - the noise, the music, the skipping, the Meeting Tree, the jam-buttie picnics, the street games they played - and a glowing tribute to her sainted mother, who seems to become younger and lovelier as Baird describes her role in teasing out the teenage John's interest in music. Julia taught him to play his first instrument, the banjo, standing behind him with her hands on his. She played the ukulele (she did a good George Formby impression) and the piano accordion, and, in the music explosion that followed the appearance of Lonnie Donegan and Elvis, she welcomed into the house umpteen friends bearing drums, washboards and rudimentary bass guitars.

Baird's book is full of lovely vignettes about the pre-Beatles period: John singing the lachrymose "Nobody's Child", the rise of The Blackjacks in their monochrome shirts and pants, the famous back-of-a-lorry gig at Woolton fête when, "The Quarrymen arrived on a lorry, and would leave that evening as half the Beatles"; the first Quarrymen gig at the Cavern, which Paul McCartney missed because he had to be at Scout camp in north Wales. Lennon's adored mother features so centrally in this chronicle of growing success that when the defining event of Lennon's life occurs - Julia was run over by a car outside Mimi's house and killed when he was 17 and his sister 11 - we feel a corresponding ache and loss, and a furious sympathy for the author and her little sister, who were kept from the funeral and not told of their mother's death for 10 weeks because they were illegitimate and shamed the family.

It has taken Baird a lifetime to put herself together after the tragedy that ruined her childhood; it's not surprising she has spent so long picking over the past, trying to tease out the family secrets and straighten out the facts. It's also not very surprising to find that she became a special needs teacher, working for many years with "excluded adolescents". Her favourite person in the world was a brother who became an excluded adolescent at 17, and disappeared forever into the big world where feelings count less than renown.

Does she think the death of his mother led indirectly to his success? "Of course. Many of his songs were chronicling his life and feelings. John said once in an interview, 'I'm not one for doing autobiography, I'd never do anything like that.' and I thought, 'John, all your songs are autobiographical.' Didn't he see it? Or did it come from depths he wasn't aware of?" Does she wish he'd never picked up the guitar? She gives a weary grimace. "Yes. Definitely. He'd be here, wouldn't he? So yes."

'Imagine This: Growing Up with my Brother John Lennon' by Julia Baird is published by Hodder, priced £18.99