How close will Lilibet be with Prince George, Princess Charlotte and Prince Louis?

It's a girl! Prince Harry and Meghan celebrate birth of Lilibet 'Lili' Diana. Video / Getty

Little does Lilibet Diana Mountbatten-Windsor know how significant her arrival into the world is to three small children across the Atlantic. For, even when you're Royal, cousins are a competitive sport.

It'll be a while before Prince George, Princess Charlotte and Prince Louis get to meet LA-born Lili, but the youngest Cambridges can now boast of no longer having three, but four cousins: Lili and older brother Archie, plus Pippa Middleton's Arthur and Grace.

But their father Prince William must be wondering what kind of relationship this next generation of Royal cousins will forge.

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Happily, the Royals have a history of close cousin bonds, presumably because no one understands the madness of one's own family quite like those inside it. Princes William and Harry have always been good pals with Princess Anne's children, Peter Phillips and Zara Tindall, who grew up near them in Gloucestershire. They also count Prince Andrew's daughters, Princesses Beatrice and Eugenie, as pseudo younger sisters. As a 7-year-old, Prince William was photographed pushing cousin Bea around in her pram.

The Queen's biographer Matthew Dennison observes: "Blood ties offer the surest guarantee of trust in a life in which little is private". Perhaps this is why Prince Charles and Prince Edward both made their cousin, Lady Sarah Chatto, Princess Margaret's daughter, a godmother to one of their children, and the Queen counted three of her cousins as her closest friends and confidantes: Margaret Rhodes, the daughter of her mother's sister, Lady Mary Bowes-Lyon; Princess Alexandra, the daughter of her uncle, Prince George, Duke of Kent; and Lady Mary Leveson-Gower, the daughter of her mother's eldest sister, Lady Rose Bowes-Lyon, who she used to play ponies with as a child.

Archie - and now Lilibet - will lead very different lives to the Cambridges. Photo / Getty Images

Surrounded by her cousins, the Queen, according to Dennison, felt she could unwind and be off duty. "Family members come closest to treating the Queen with any degree of normality," he says. But the cosy cousin bonds within the Royal family also have a lot to do with the sizeable chunks of time they spend together – something Archie and Lili will miss out on, given they live 8,787km away on America's West Coast.

Margaret Rhodes, who was one of the Queen's bridesmaids, spent all her summer holidays with her at Balmoral and even lived with her at Buckingham Palace and Windsor Castle during the World War II.

Prince William and Harry's generation of Royal cousins haven't lived in such close quarters as this, but they've still had geography on their side, spending Christmases together at Sandringham, holidays at Balmoral and larking around at polo matches and parties in London and the Cotswolds. The little Sussexes, raised in the privacy of palms in Santa Barbara, will miss out on all these family occasions, as well as the starchier Royal occasions such as Trooping of the Colour, where they would ordinarily have bonded (in silence) with the rest of the Royal "brat pack".

The Royals have a history of close cousin bonds. Photo / Getty Images

There'll also be none of the shared experience of Royal duty among the Sussex-Cambridge cousins. Royal cousins tend to see it as their obligation to support the monarch and heirs – a back-scratching exercise; their loyalty and hard work in return for a Royal property or two and an income. Margaret Rhodes, for example, lived on the Great Windsor Estate until her death in 2016; Princess Alexandra has use of a grace-and-favour apartment at St James's Palace.

Even the York sisters, who aren't officially working Royals, and thus don't receive a salary from the Royal household, are dutiful cousins, singing for their princess status by supporting the Queen, Prince Charles and Prince William at official events and being patrons of various charitable trusts. Without titles, or a Royal income on the horizon, the Sussex children are unlikely to feel any degree of subservience or obligation to Prince George and his siblings – one would imagine they'd sooner support their parents' charitable endeavours in America, which might resonate more with them than stuffy British institutions.

Zara and Mike Tindall are known for being down to earth. Photo / Getty Images

This discrepancy in status, while galling for their parents Harry and Meghan, doesn't mean, however, that the Sussexes won't become good buddies with their titled cousins. The fact that Zara Tindall and Peter Philips do not have titles – a choice made by their mother, the Princess Royal – has never affected their relationship with Princes William and Harry. In fact, the "normcore" cousins have always provided a hit of ordinariness for the princes, exposing them to a down-to-earth horsey social scene in Gloucestershire that they wouldn't otherwise have encountered.

As semi-outsiders, Tindall, who is Prince George's godmother, and Phillips are grounded confidantes, who will quickly step in to bring their princely cousins down a peg if they get above themselves. Their children – Peter has two with ex-wife Autumn Phillips, and Zara has three – see it as their duty to do the same to the younger Cambridges; while watching a match at Beaufort Polo Club a few summers ago, Phillips' daughter Savannah was told off by her mother, Autumn Phillips, for pushing Prince George down a slope.

Being raised in La La Land, it's unlikely that Archie and Lili will be able to offer a similar degree of normality – or any normality at all – but they'll introduce their British cousins to the Californian way of life. Young George, Charlotte and Louis may yet enjoy sun-soaked days by the pool with Hollywood royalty, while getting a feel for Harry and Meghan's approach to public service. Their teenage nights out in LA with Archie and Lili will be straight out of Clueless, but they'll find their cousins are more clued up than they are when it comes to mindfulness, waffles and the most secluded SoCal beaches.

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Meanwhile, for the young Sussexes, holidays in England with the Cambridge cousins will provide a fascinating window into the world in which they would have lived if their parents hadn't jumped ship: never-ending church services wearing Sunday best, Pony Club and prep school pranks in the nursery; Prince William terrified his cousins Bea and Eugenie one Easter by biting the head off a sugar mouse. They'll have their resilience tested on windswept Norfolk beaches and wintery walks at Anmer Hall and, as teenagers, Prince George will haul them off to Prince Harry's old Chelsea drinking dens on the King's Road, perhaps with Princess Eugenie's son August and Princess Beatrice's yet-to-be-born sprog in tow.

The question is, who will lead whom astray – the Cambridges from Kensington or the Sussexes from Santa Barbara? Anyone who watched the BBC's adaption of Nancy Mitford's In Pursuit of Love will know that this is the whole point of a cousin.

Prince William recently opened up about a particular incident where he and Peter Phillips herded Zara into a lamppost, earning themselves a ticking off from the Queen. "We were chasing Zara around, who was on a go-cart," he explained on a Sky documentary. "The lamppost came down and nearly squashed her, and I remember my grandmother being the first person out at Balmoral running across the lawn in her kilt."

With a generation of transatlantic Royal grandchildren to contend with, Prince Charles is going to have his work cut out.