The summer my cousin arrived was the summer that everything changed. Golden months that were both an ending and a beginning.

It was early June when my mother received the fateful letter. At least that was how my sister Juliet described it.

"Your aunt writes to say that Iolanthe would like to come and stay with us for a few weeks," my mother said over breakfast. We still had a Saturday morning post in those days.

My father raised his eyebrows but said nothing. He had delivered a baby in the early hours of the morning and looked tired as he drank his coffee.

We hadn't seen Iolanthe for several years. I suppose I couldn't have been more than seven or eight the last time, which made it nearly a decade ago. I remembered a visit to our late grandparents, and a blonde girl who was very smartly dressed and self-possessed. Juliet and I were all over the place in those days, climbing trees and tearing our clothes, but Io had already transitioned to being a "young lady" as they used to call it. Even from an early age she had seemed more a part of the grown-ups' world than our world.

William, my elder brother, regarded the news with some contempt. "What on earth would she want to come here for? It's hardly the Riviera." William was in the second year of medical college, following in our father's footsteps, and tended to regard our aunt and her daughter as frivolous.

In fairness we mainly received their news through various postcards, which rather gave the impression that they were always travelling and jaunting about. But Aunt Pia was an opera singer and had to travel for work. Io had just spent three years studying music and was supposed to be following in her mother's footsteps, or so we had been told.

William, who considered his planned career to be a matter of life and death and far more serious than singing, had little regard for our aunt and her way of life. "She can hardly occupy the spare room for the entire summer. What if Granny wants to come and stay?" he pointed out, referring to our paternal grandmother.

The problem was that while our house was large, we were also a very large family. There were five of us siblings, the "doctor's children" as we were always referred to. First William, then Juliet, who had just finished her A-levels. Then me, with one more year of school to go. Next came Alice, three years younger than me, and finally Toby who was ten.

My mother drew her brows together. "She could share with you two, I suppose," she suggested, looking at Juliet and me. But even as she said it, she recognised how impossible it would be. Iolanthe, sophisticated and already well-travelled by the age of twenty-two, could hardly be expected to slum it with two schoolgirls. Squeezing a third bed into our room would also be difficult. Alice, like Toby, had a small attic room so that wasn't an option either.

"We'll just have to do up the box room, then," Juliet said. She was always the practical one among us.

William glared at her. He had had designs of making that room into some kind of study for himself. Given he was away all term time in another city it hardly seemed fair of him to end up with a suite of rooms. But since he had made no progress in clearing it out so far, there had seemed to be no point complaining.

"That's settled then," our mother said. "We'll clear it out and have a lovely bonfire." She loved bonfires. She still took a childlike delight in certain eclectic things, despite being a doctor's wife and a mother of five.

"I wonder what she's like now?" Juliet said later, when we were back in our room. She stood in front of the mirror making faces at herself and pouting. She was trying to will her lips to grow fuller, since she had read in one of our magazines that "bee stung lips" were the height of fashion.

"Very glamorous, I should think," I said. "I expect she'll find us all dreadfully dull and provincial."

"I wish we could go and stay with her and Aunt Pia instead," Juliet said. "Monaco or Paris or Salzburg somewhere, all those places that they endlessly go to. Think of all those French boys."

"You wouldn't get French boys in Salzburg, they'd be Austrian," I pointed out.

Juliet was drawing around the edge of her mouth with a lip pencil, still trying to give herself a pout like Marilyn Monroe in the black-and-white poster on the wall next to the dressing table. My side of the room was covered with posters of vintage Hollywood, a passion of mine. "Austrian would do just fine. Anything other than the useless specimens around here," Juliet said.

"They'll probably all fall for Io, if she's still pretty. Anyone new always gets the attention."

"She's welcome to anyone around here," Juliet said. "I can't imagine the village will provide very rich pickings for her."

We were very wrong about this.

Then Io arrived. We picked her up from the station, my mother, William, Juliet and I. My father had surgery on alternate Saturdays so didn't accompany us.

I read somewhere that when a modern audience was shown an early Gary Cooper film, they gasped at the first images of him because of his sheer beauty. I think we all felt a bit like that when Io arrived. She had that same vintage Hollywood glamour, those looks that seem to light up a scene, even though she was thoroughly modern.

Everything from the way she stepped off the train to how she greeted my mother and the rest of us is preserved in my mind like a film. What did she look like? It's hard to describe, but if you can imagine Gene Tierney with ash blonde hair and violet eyes, she was something like that.

Even William, who had feigned reluctance about coming to the station but was secretly as curious as the rest of us, got over most of his antipathy at the first sight of her. He even insisted she took the front passenger seat which was always his privilege when my mother drove.

"I'm happy to go in the back, you're far taller than me and must need the legroom," Io told him, but William insisted. He even opened the door for her, which I'm quite sure was the first time he had done so for anyone except our grandmother.

Io seemed genuinely ravished, as she put it, to see us all. She was much more vibrant than the well-behaved but languid sixteen-year-old I remembered her to have been. On the short drive home my mother asked her the usual polite questions about the journey, and Io regaled us with tales of the other passengers. Everything she said was spellbinding. She made British Rail sound like the Orient Express.

When she got out of the car she tripped on the loose stone in the driveway, and stumbled and nearly fell. She burst out laughing and suddenly appeared more human. We were all just so overawed by her. Until that point she hardly seemed real.

I remember the scene when we got home. Alice became shy and Toby stood there staring as though Io was some kind of alien species. We all entered the dining room, William carrying her luggage, and my mother going to the kitchen to put the kettle on. I think she needed a few moments alone to get her head around it all.

Io had brought presents for us all. For my mother there was a huge box of calissons from a recent trip to Provence and a bottle of Jean Patou's Joy. It was my mother's favourite scent but it was terribly expensive, so she used it very sparingly, eking out every last drop. Her last bottle was nearly empty so Io's gift was a real delight.

For William there was a smoking pipe set in a beautiful wooden box. "I'm sure it's terribly bad for your lungs what with being a doctor, but all the young men about town seemed to have taken it up," Io told him.

In one fell swoop she had conquered him utterly. She had bestowed on him an adult gift, referred to him as a doctor and promoted him to the league of "young men about town". I could see William's chest practically swell at this. I feared he was going to be more pompous and condescending than ever towards the rest of us.

For Juliet, Alice and me there were colourful silk scarves. We were thrilled with these because there was a fashion at that time to tie a scarf around your ponytail, and these were far nicer than anything they sold in town.

There was a bottle of Armagnac for my father and a huge jar of sweets for Toby. It was like Christmas in June.

"You can come and stay every year," Toby said, and disappeared to hide his sweets in one of his dens.

When my mother had made the tea, we sat around the table drinking it. Even Juliet didn't really know what to say. Fortunately Io chattered happily away to my mother, mentioning various old family friends, what Aunt Pia was up to, and expressing how lovely it was to see us all again.

"I only regret I left it so long," she said. When she said it she looked like she genuinely meant it.

She tried to include us in the conversation when she could. "Aunty Sophy - you might remember her from Grandfather's funeral" or "Ann Peabody - she was at school with my mother and yours". Though we didn't really know or remember any of these people it was still a delight to listen to it all.

Alice had made a cake especially for the occasion and I had a bad feeling about it because Io didn't look like the kind of girl who would eat much cake. I didn't want Alice's feelings to be hurt. But Io accepted a huge slice, ate all of it impressively quickly, and offered glowing praise to the baker.

This was enough to win Alice over.

When we showed her upstairs, she claimed to adore the box room that had been fitted out for her. We had been worried it was too cramped and dark, for it had a funny round window. We had put a vase of flowers in there to brighten it up. "It's perfect," Io told us.

"Sorry it's a bit small," Juliet said. "There are so many of us and Granny Lawrence has to have the spare room when she visits because she can't manage too many stairs."

"I love it. I expect I'll be spending more time gossiping with you both in your room anyway," Io said.

We could only hope so.

Aunt Pia - whose real name was Persis - still retained her maiden name of Clermont. She had passed it to Iolanthe since she had never married Io's father. He was a racing driver called Maurice Jones who had died in a crash before his daughter was born.

The Clermonts, originally the de Clermonts, were an aristocratic Anglo Norman clan who had dwindled in wealth and influence over the centuries. There had been a Lord de Clermont back in the seventeenth or eighteenth century but he had been stripped of his estates as a result of some scandal. Since then there had been no stately pile, no position at court, just the family name of which Io was now the last and youngest bearer in our branch of the family.

There were some distant cousins of my mother and Aunt Pia who had managed to breed a new generation of male Clermonts, but we had never met them and didn't even know their first names.

My father had once described the Clermonts as a "dissolute and disreputable clan" and my mother had not disagreed. From every branch of the family tree hung an assortment of dilettantes, tricksters and downright rogues. According to legend there was even a highwayman who had fled overseas to escape a hanging. They were a veritable flock of black sheep. My grandparents, who had been eminently respectable, were anomalies.

I suppose the earlier de Clermonts had to be artful to survive when all their property was taken from them, and the trait had passed down to subsequent generations. I was secretly rather proud of our family history.

With Io there, I started noticing increasing glimpses of Clermont blood in my mother. She had always seemed the absolute opposite of Aunt Pia and her bohemian ways. But through Io I got a sense of what the two sisters must have been like in their younger days. Theirs had been a life of high society, parties and gadding about with fascinating people. How my mother had managed to sensibly marry a doctor and settle down to raise five children in a quiet village was mystifying, Io said.

"She was the beauty of the family. They all thought she might go on the stage, you know. Then she met your father," Io told me.

This shocked me, for we had always assumed that Aunt Pia was the greater beauty. She was certainly the most glamorous of the two of them.

Io dug out a black and white snapshot from her handbag. "I had a copy made of this, I've always loved it. They look so free and full of expectations, on the verge of it all."

It showed my mother and Persis standing on a lawn, both in long gowns, with their hair set in an old fashioned style. They were both dazzling.

"What were they all dressed up for?" Juliet asked.

"Some garden party. But see how lovely they both look? They made a sensation, I imagine, the two divine Clermont sisters stepping out into society. Just like you two must do," Io said to Juliet and me.

We fell about laughing at this. I couldn't think of any occasion where Juliet and I would have been dressed up to the nines and posing in such a way on a lawn, let alone making a sensation.

"I'm quite serious," Io said. "You're both stunning. You're all a very good looking family. What with your mother and your father - who is such a handsome man - you were hardly going to end up plain Janes, were you?"

To some extent we knew this was true because people did remark on it. Toby had had golden curls as an infant and people had stopped in the street to admire him. We had also heard ourselves referred to as "pretty girls" and William as a "handsome young man" - he hated this, so of course we teased him mercilessly about it for weeks. But we never took it very seriously. We certainly didn't have Io's spectacular degree of style and loveliness.

It feels vain relating all this, but that's how it was, and that's what Io said to us. Though we were all quite different: William had our father's auburn hair, Juliet had thick and wavy chestnut locks and our father's hazel eyes. I ended up dark blonde and Alice stayed fair like our mother. Toby's golden cherub curls darkened to a similar shade to Juliet's, but he had our mother's blue eyes. It was an interesting mix of genes.

But none of us could truly compare to Io. Yet the more we got to know her, the more we realised her appeal was far more than her looks. It was her charm that made her dazzle, a charm which derived from a sheer delight in life. She loved people. Everything was an adventure to her, everything entertained her. She was never bored.

I wondered what it must be like to be that way, and I envied it more than her looks. We were more proud than resentful of her beauty, it was a family asset to stun and impress our friends and neighbours with. We had no hope of competing with it, so we embraced it.