They have four reality shows in rotation on E! and they fight in one way or another on all of them, whether it's over a man, a perfume deal, an engagement or a baby. Yet at the end of the day, those three raven-haired American-Armenian Kardashian sisters are each other's biggest fans and fiercest defenders.
Perhaps they are exceptional, but even when one sister is famous and the other clearly has managed to carve out her own path, it's a cause for wonder. One can't help look at Penélope Cruz and her sister Monica, or Lily Allen and Sarah Owen - particularly in the light of their recent vintage clothing shop closure - and think, really? You are OK with all of this?
All eyes were on the maid of honour Pippa Middleton when she swept up to help her elder sister Catherine out of the Rolls-Royce as she arrived at Westminster Abbey in London for her wedding to Prince William. Amid the deafening screams of the crowd and the lenses of a thousand cameras aimed in their direction, as she passed Kate her lily of the valley bouquet, Pippa said: "You look amazing."
It wasn't a particularly remarkable or profound statement. Pippa was only voicing what we were all thinking. But the fact that she thought to compliment and reassure her big sister at that moment, amid all the pomp and hoopla, spoke volumes about their relationship. It is perhaps no surprise, therefore, that Kate is said to refer to Pippa as her best friend.
As relationships go, sisterhood can be one of the most confounding and complicated. Female siblings can simmer with jealousy, bitterness and even contempt.
"I don't believe an accident of birth makes people sisters or brothers," said the American writer Maya Angelou. "It makes them siblings, gives them mutuality of parentage. Sisterhood and brotherhood is a condition people have to work at."
As someone with two sisters (and a brother), to whom I am very close, I am lucky enough to be able to imagine what a huge support it must be for Kate to have Pippa. My sisters are the first people I will turn to if I need help. We are very alike, share similar interests and speak on a regular basis. When we were young, the three of us, together with my brother, would always present a united front against my parents should any one of us need defending. "Nope, I've never seen her buy a packet of cigarettes," we would all swear, in the face of indisputable evidence. "In fact, she hates smoking. Hates it."
Of course, children squabble, and we were no different. On my sixth birthday, as I was presented with my shiny new bicycle, my older sister leant over and whispered: "It's just my old bike that they've had repainted." You could practically hear the slow hiss as my bubble of joy deflated. Equally, in my turn one Christmas, when my younger sister was having way too much fun imaging all the presents Father Christmas would bring her, I kindly exploded that myth for her.
Still, it was a contrast to what I encountered at boarding school, a situation that echoed Angelou's words. One pair of siblings there would have a monumental catfight over who should read any letter that arrived from their mother - although it was always addressed to both. If one managed to swipe it before the other noticed, her belongings would be viciously ransacked until it was found. Twenty years on, they are not on speaking terms. They did not even invite each other to their weddings.
Happily, that seems to be the exception. "There are only 11 months between me and my younger sister," says Sara Al Aslai, a business tourism executive at the Abu Dhabi Tourism Authority, who has seven sisters and three brothers. "We were always together, so we did fight when we were little. It's different now we're older, though. We're very close. Sometimes, I will go shopping on my own. We will both come home and found we've bought the same thing in the same colour."
Like the Middleton girls, Al Aslai describes her sister as her best friend. "When I am out with my friends, I will call her and my friends will say, 'Didn't you just speak to her?' and I'll reply, 'Yes, but I have something new to tell her.'"
A healthy sense of competition, is, more often that not, par for the course among sisters; something that is taken to the extreme in cases where they have chosen to operate in the same field, often with differing levels of success. Venus and Serena Williams, though fairly evenly matched, have faced each other in 28 professional tennis matches since 1998. Dannii Minogue had to content herself with the odd television-presenting spot after her music career nosedived, while her older sister Kylie's hits dominated the pop charts during the 1990s. And the singer Solange Knowles has not achieved nearly the same level of commercial success as her elder sister Beyoncé.
Choosing separate interests was what kept Emma Canwell, a lawyer based in Abu Dhabi, and her younger sister Lucy close, even during their tricky teenage years. "We are two years apart and we're both quite competitive people," she says. "Our personalities are totally different, but we never competed with each other because we were lucky enough to be interested in totally different things. Whereas she went down the sports and science road, I did art and music."
Being such a tight unit can have its drawbacks, though, when it comes to other relationships. "When she was funny about a boyfriend of mine," says Canwell, "and said she wasn't sure, I found it really hard to carry on the relationship knowing my sister didn't like him. My family has always been really important to me and I didn't want to be with anyone who didn't get on with them."
Even now, Canwell says, her husband sometimes struggles with the intensity of her relationship with her sister.
"I think one thing any husband of close sisters has to be prepared for is the fact that we will talk about everything." she says. "I probably wouldn't tell my parents if I'd had a big fight with my husband but I would automatically tell my sister. It's nice to get it off my chest and she's not going to judge him like my parents would, she's just going to make me feel better about it."