We are dazzled by sisters. We are drawn to them and fascinated by them.
The interaction, the power play, the similarities undercut by idiosyncrasies. The shared grace and beauty; the identical tempers; a moment of shock when their faces drop and they look once more like the toddlers they used to be, staring out from underneath home-cut fringes and fighting over an iced bun.
The sororal bond is the most intricate and delicate human relationship there is. We spend years trawling through friends and acquaintances, blind dates and one-night stands for suitable life partners. But we rarely stop to think about the longest, most intimate and committed union that we're already in: the one we entered into at birth with our siblings.
Of course, there is much discussion to be had about the sparks that fly between brothers – the fizzing, competitive machismo between the likes of the Milibands or Jedward ("He got the leadership!" you can imagine the cry going up. "He got more of his name into our portmanteau!")
But there is something singular about the overwhelming affection and sometimes uneasy tension between sisters, something born of the female tendency to be more sociable (sisters tend to maintain more contact than any other sibling pair) and of the subconscious competition that arises among women – studies show that we are more likely to be envious of those with whom we identify and share character traits.
Then there's the almost mythical, ferocious intensity of feelings that women can nurture, as well as the classical trope of sisters either looking after or warring with each other. The mythologising of sisters, as best friends or sworn enemies, has at its heart the general anxiety of outsiders unable to fathom the complexity of the relationship – it's no coincidence that the ancient Greek Gorgons, reptilian females who could turn men to stone with one look, were three sisters. I've seen it happen in clubs, when heroic men approach a clutch of women – who all have the same bone structure and are deep in conversation – and crumble under their gaze like so much lithic matter.
Yet people are still intrigued, whether they're jealous or simply lured by these similar-faced sirens. Sisters are everywhere. From the Olsen twins, who became famous as child stars and earlier this month narrowly missed out on a major award for their fashion label The Row, to Kate and Pippa Middleton, beaming their automaton smiles down on the masses and flicking their rich-girl hair. Sisters, their impetuous camaraderie and tempestuous flare-ups, have become the ultimate marketing strategy. We can't get enough of them.
I am one of three sisters, younger than No 2 by nine years and 14 years behind No 1. It was like having three mums growing up, I tell people who remark on the age gap. But it wasn't really – because your mum wouldn't record you singing pop songs you'd composed yourself or balance you in mid-air on her upturned feet so you knew what it would feel like to fly. While mums always have the best advice – namely, don't do that thing that might turn out badly – sisters have the best rationale, because they did that thing that then turned out badly and they don't want you to do it, too.
"Many people with sisters feel that they don't know what they'd do without them," says Dr Terri Apter, who teaches at Cambridge University and wrote The Sister Knot.
"But it's harder for sisters to get along than brothers, because men are socialised to accept competition as part of any relationship. It doesn't feel out of keeping for them, while sisters – and women – can be very troubled by that competition."
One of the most well-known sisterly feuds is the eight-decade stand-off between Hollywood grandes dames Olivia de Havilland and Joan Fontaine. Born a year apart, they set out to make their careers in the movie industry during its golden age and were the first sisters to win Oscars and the first sisters to compete for the Best Actress statuette – which Fontaine won in 1941 for her role in Alfred Hitchcock's Suspicion. When Ginger Rogers read out her name, Fontaine said, all "the hair-pulling, the savage wrestling matches, the time Olivia fractured my collar bone, all came rushing back... I felt age four, being confronted by my older sister... I had incurred her wrath again."
The enmity deepened when, after cancellations and poor planning, Fontaine was the one to present de Havilland's Oscar in 1946. She refused even to shake Fontaine's hand and the sisters do not speak to this day.
At a party to celebrate the 100th anniversary of Bette Davis's birth in 2008, Fontaine was a no-show, despite having previously confirmed her attendance, because de Havilland was also going.
"We have powerful expectations of loyalty from a sister," the sociologist Marcia Millman says in her book The Perfect Sister: What Draws Us Together, What Drives Us Apart. "But along with the idealised image of sisters, that they are always close, there is a stereotype that sisters are very competitive. It's the two extreme." It has become increasingly common for those in the public eye to turn to their sisters for help.
Margaret Thatcher's best friend was her sister Muriel. Beyoncé Knowles's little sister, Solange, supported her on tour; Pippa will be Kate's right-hand woman, it is understood, while Kylie and Dannii Minogue regularly help each other out with their respective projects. Now that their careers have levelled out, they are considered in their own right; time was, neither could enjoy success without speculation that the other was exasperated or, worse, estranged.
When the popstar Lily Allen retired from music to set up her own vintage-clothing boutique last year, it was her wildchild older sister Sarah Owen who became her business partner. "Working with my sister is a more extreme experience than working with anyone else," Owen says. "The highs are more fulfilling and enjoyable but the lows can be complicated and hard to navigate, with ultimately much more at stake.
"The bond between Lily and myself is ever-changing but always present. I don't take it for granted, but at the same time, I'm secure in the knowledge it will always be there, in one form or another."
Perhaps it's because there's so much at stake that tensions can run so high between sisters; each utterance or action is impassioned when it comes to relating to one another. A phone call never starts with "hi, how's things?" but "I haven't seeeeeen you for so long!"; a new dress never goes unnoticed without a compliment; career highs and lows are noted with either glee or a resoundingly blunt critique. It's just like that – which is why it's fascinating to try to imagine how celebrity sisters react to one another. Did Kate Middleton have a go at Pippa for stealing the limelight with her much-admired derriere? How did she feel to see Pippa mock-crowned on the cover of a popular women's magazine the week after the wedding? It's easy to imagine them picking over it one Christmas at Balmoral in 15 years.
We all love speculating about the private lives of our idols and because the sister bond is something many of us can understand, it's infinitely easier to project our own experiences on to them. But why do we care?
"On a simple level, it's just 'oh, so there's another one?'" the celebrity writer Paul Flynn says. "But underneath that is the assumption of a unique undercurrent of female solidarity. Family antagonism is easy – and fun – to reduce to tabloid thinking. But I'd warn any celebrity sister act against giving off a display of anything other than complete cordiality – betraying your sister is the ultimate public moral no-no."
Sisters provide all the dramatic ups and downs we require from our tabloids without the dangers of homes being wrecked or marriages ending; we recognise that siblings fall out with each other again and again. It's part of the allure of LA sororal dynasties such as the Hiltons and the Kardashians, whose antics are never far from the TV screens and gutter press. "They're the spray-tan revamps of Jane Austen's Bennets," Flynn says. "Same story, different styling."
Kim Kardashian, who rose to fame as part of a family reality show based on her and her sisters, launched her own brand of perfume in London last week. "I love working with my sisters," she says. "We're so lucky to have made a career of doing things together. Of course we argue, like all sisters do, but they are my best friends.
"We're all so different – a lot of people have said that's why our TV series does so well, as every girl can relate to one of us."
It's true that sisters take on roles according to their place in the line-up and this only adds to our sense of them as sets of walking, talking dramatis personae. "Generally speaking, older sisters are more rational, logical and determined," says Sheri Nicholson, clinical director of The Harley Therapy Group. "They're successful and motivated goal-seekers. Younger sisters are a bit more relaxed. They're creative, artsy and sociable."
And there are many other stereotypes that anyone with sisters recognises – the youngest is, of course, the cosseted baby, while the eldest looks after things. The middle sister is the negotiator, the peaceful, kind one and (whisper it) the one who got all the allergies. These roles define who we are when we're not around our sisters, too – I blame their competence at looking after me for the fact I am almost incapable of looking after myself; their doting attentions when I was young means I am simply no good at being on my own.
"As the youngest, I have always had to try a bit harder to make my voice heard," says Sofia Malm, the youngest of the three sisters who set up the fashion label Dagmar. "I think the older I become, the closer we get and the more alike and equal we are in every way."
For every celebrity lifestyle one marvels at, it seems so much stranger that two people in the same family could possibly achieve it. And from there comes our joyous scrutiny of the balance of success, fame and fortune. Because it doesn't seem possible that two people so close should have so much – or that it should be distributed evenly.
For every sister that makes it, of course, there are more who don't and are often carried. The failed pop careers of Jamie Lynn Spears and Ashlee Simpson, for example, younger sisters of Britney and Jessica; Posh Spice's sister, every inch as glamorous and almost as recognisable, but terribly normal. It's a difficult situation in which to find oneself and one fraught with bitterness and resentment.
"With sisters, you learn to love and hate the same person," Terri Apter says. "You have a companion and an ally, someone with whom you can identify and really love, but they're also a competitor. And it's difficult, growing up, because they can hoover up all the attention you feel you need for yourself."
It's the same difficulty that all children grapple with when faced with sharing a parent or favourite toy with some newly arrived upstart of a little brother or sister. But something special kicks in as girls mature together and grow closer, something that is utterly unique and incomprehensible to anyone beyond the emotional bond. It's innate and immediate, instinctive and a priori, cultish even – a world of private jokes and knowing what the other is thinking, of cackling and crying together, of sharing tall stories and soothing war wounds.
There is simply nothing like it. And if you're lucky enough to be born with it, it will never leave you; with sisters, you will never be on your own.