She really is the sister William never had ...

PRINCE George and Mia Tindall couldn’t have looked happier and more relaxed in each other’s company. The five-year-old second cousins have clearly inherited the easy familiarity of the life-long friendship of Prince William and his cousin, Zara.

So much so, it is understood that the Tindalls were staying with William and Kate at Anmer Hall, their Norfolk retreat, last weekend while leading equestrian Zara was competing in the Burnham Market Horse trials, where the charming photographs were taken. No-nonsense Zara, 37, daughter of Princess Anne, has always been the fun cousin whom William can rely on to lift his spirits in dark times.

And, as we know, he has his fair share of them. It would not be an understatement to say that she has been the big sister he never had. She has, of course, always been close to Harry too (he is believed to have been recently made Lena’s godfather).

One friend recalls a country dinner party where Harry was left in hysterics as he showed his mobile phone to his host boasting a barrage of expletive-ridden jokey messages from his cousin which had popped up between courses.

Both could, of course, party with the best of them in their time.

William, who is believed to be Mia’s godfather (though it has never been confirmed), has always enjoyed the same free and easy friendship with Zara, over and above their family ties — whether it be larking around at the polo, arms slung with easy intimacy around the other’s shoulders, or chatting away in the background at a major royal event such as Trooping The Colour.

That’s not to say it was a given. The Queen’s own children have famously not always had the easiest of relationships.

Their upbringings were very different: Zara, unfettered by any official royal role, has always lived at Gatcombe, her mother’s Gloucestershire estate while William was based principally at his mother’s residence in Kensington Palace.

Both attended different schools and universities and have followed markedly different career paths.

But they do meet up, sources say, over and above official family events and keep in touch by phone and WhatsApp — jokey little messages, family news and the like.

Theirs is a unique bond, forged by circumstance as well as blood, that few outside of the Royal Family could ever understand.

What’s the betting that one day we see cheeky little Mia, with George and Charlotte in tow, ripping up the royal rule book together as their laughing parents look on?