By any reckoning, last month Kate Middleton should have been enthroned as Britain's unrivalled icon of style.
Yet, after that glorious wedding, whose face adorns the cover of no fewer than five celebrity and fashion magazines, from Grazia and Heat to Hello!? It's not Kate's, but Pippa's.
Even at the wedding, it was Kate's head that wore the tiara, but Pippa's bottom that caused an internet sensation.
Then there were the leaked holiday snaps on a boat in Ibiza, taken in 2006, that invited even more praise for Pippa's perfect posterior.
Aside from admiring the Middleton sisters' fabulous figures in their white bikinis, you might have spotted that while Pippa dived in a graceful arc backwards off the yacht, Kate watched from the sidelines, keeping a tight grip on the safety rail.
Whisper it, but it would seem that Her Royal Hotness has stolen more than a little of her older sister's limelight.
In the House of Windsor, you don't have to look far to find another naughty little sister who is sexier, prettier and more red-blooded than her staid older sibling. Princess Eugenie, an archetypal younger sister, startled her teachers by streaking under influence through Marlborough College's shrubbery just before she was due to take her A-levels.
Poles apart
"I am definitely not as polite as Beatrice," she has admitted of her relationship with her more self-controlled sister. "At a party, I am much louder while she is far more solicitous."
Then the trend of the little sister being sexier, more fun-loving and attention-seeking than her elder sibling is one I have watched play out in different scenarios all my life.
I call it Little Sister Syndrome — an affliction that sees many girls ready to use a big sister's success to gain notoriety themselves.
Just look at X Factor's Dannii Minogue, who is now arguably a bigger name in Britain than her older sister Kylie.
Then there's tennis star Serena Williams, a younger, more aggressive sister who beat her older sibling at her own game.
Georgia Jagger has been seriously upstaging her sister Lizzie of late — so much so that Lizzie was forced to take drastic measures to put her little sister spectacularly back in the shadows by agreeing to a raunchy Playboy shoot.
Nor is Little Sister Syndrome confined to celebrities. At 38, I am two-and-a-half years younger than my sister, Lydia, and just as Serena pursued Venus across the world's centre courts, I tagged after her into journalism.
After university (same university, same course), I was languishing in a dusty warehouse at Christie's auctioneers, cataloguing furniture and watching from the sidelines as she was having a ball at the Cannes Film Festival.
From where I was standing, she was having a lot more fun. Just like Pippa, I wanted a piece of my big sister's action.
And just like Pippa, this found its most unedifying — and unintentional — expression at my big sister's wedding.
As a bridesmaid, I was supposed to be reading an affecting poem written by my great-uncle to the assembled crowd of family and friends. I stood up, took one look at the expectant faces and burst into noisy sobs.
The ceremony ground to a halt while I recovered and the photographer went wild capturing the drama. It wasn't a deliberate ploy and I was mortified, but for ten minutes, her big day was mine, too.
That's the thing with little sisters, though. They just can't help themselves.
The theory
As Professor Nigel Nicholson, whose book Family Wars deals with conflict within families, observes: "There is a theory about birth order and rebellion that says eldest children are motivated to conform to what their parents want and fit an orthodox model because they believe that, if they behave, their parents will love them. Along comes a second sibling, who sees the oldest has cornered the market in parental co-operation, so they rebel to secure their own individual angle."
A novelist friend recognises this behaviour instantly and, being the youngest of five, she should know.
"As a younger sister, you can easily feel that you are not going to be heard," she says.
"That can be a strong dynamic: people tell you to pipe down or ignore you and you feel that nothing you say has value. You can either believe that, or the worm will turn and you claim your inner fabulousness. At that point, you might just upstage your sister on her wedding day."
The realisation
Nicholson explains how Pippa Middleton may have realised that being "pretty and pleasing would be as effective a way of attracting attention as being merely well-behaved". She has certainly succeeded in pleasing large swathes of the internet.
It is interesting to note that while we have pictures of Pippa cavorting in deep orange tan and pink mini-skirt, or wearing nothing but a few sheets of tidily arranged loo roll at a party, there are fewer of Kate that show her having unbridled fun.
My theory is that Little Sister Syndrome is, in fact, an expression of how much we rely on our older role model. They give us the security to be a little more reckless and carefree. As the little sister, I know I breeze through life assuming everything will be all right in the end. And secretly I know that, even if it isn't, big sister will pick up the pieces.
The measures that psychologists use to identify risk-takers include whether they are unfaithful, have a clean driving licence, gamble, have tattoos and commit anti-social behaviour.
While I don't have any tattoos and am not a criminal, I certainly drive a good deal faster than she does. But then if we all drove like her, no one would ever get anywhere.
In short, my more rackety personality has frequently left not only my big sister but my mother and my husband (all older siblings, incidentally) prey to sleepless nights. Most little sisters I spoke to were happy to admit they are the family tearaways.
Not as perfect
"I don't think I'm bad, but I am naughtier than her," my 21-year-old sister-in-law Fiona Hill, a drama student, says of her 24-year-old sister, another Pippa, who is a primary school teacher.
"I never wanted to be good because she was the good one. She has always been called Perfect Pippa. I wanted to be cool and fun and silly, the drama queen of the family. Our mother calls me The Wild One," she adds.
Big sister Pippa recalls: "I had to work to get things — having my ears pierced, being allowed a mobile phone, staying up late. Fiona had those things a lot earlier.
"On top of that," she adds dryly, "she used to be the one dancing on tables and hiding from the teachers. Mum found Fifi funny because, as a parent, she was more relaxed by that time — but if it had been me she would have been panicking."
So what's a big sister to do, to tame these reckless younger rivals?
In Kate Middleton's case, the answer is obvious, for there's another naughty younger sibling who would be more than a match for Pippa.
Judging by the pictures of Prince Harry ogling his sister-in-law on the Big Day, it's not such a far-fetched scenario. Might we one day see Her Royal Hotness exchanging a kiss with His Royal Rebelliousness on the Buckingham Palace balcony? Now that would certainly be a double whammy to make Carole Middleton burst with pride.
