Prince George, Charlotte, and Louis share adorable sibling moment in unseen snap - and the royal protocol rule you might have missed

Kate Middleton and Prince William have released an unseen picture of their three children, Prince George, Princess Charlotte, and Prince Louis, and the young royals not only look adorable together, but the picture also shows off an important royal protocol rule. 

To mark Christmas this year, after releasing their Christmas card earlier this month, the Prince and Princess of Wales shared an unreleased picture of their three children taken on the same day as their Christmas portrait. 

The sweet photo shows Prince George, Princess Charlotte and Prince Louis, who sit second, third and fourth in the royal line of succession respectively, smiling with their arms around each others' shoulders in their matching white shirts and casual jeans. 

The parents shared the photo alongside the caption, "Wishing everyone a very Merry Christmas, from our family to yours! W & C."

Fans quickly took to the comments, gushing over the young royals. One user wrote, "Absolutely gorgeous photo. Merry Christmas," while another said, "Such a cute trio! I love to see them just being them!"

The photographer who shot the sweet picture also shared the snap on Instagram, using the caption to reveal some insight into the photoshoot process with the young royals. Josh Shinner shared, "Hope you all have a brilliant day and that your cracker jokes are as good as the knock-knock ones I picked up from these three..."

In the photo, Prince Louis is the only sibling wearing shorts. That's because it's royal protocol for him to do so - why? Because he's only five and royals have to hit a certain age before they're allowed to step out in trousers.

“Boys wear short trousers until they are eight,” Ingrid Seward, editor of Majesty magazine, told People. “It is shorts until you’re eight and then ‘woo, you’re in long trousers’ … They suddenly feel very grown up.”

The photo is just another one of many insights we've had recently into the sibling bond between George, Charlotte and Louis. And while, in his memoir Spare, Prince Harry voiced concerns about his niece and nephews and their relationships with one another, it seems he was worried for no reason. 

One significantly poignant moment between the children came during King Charles' Coronation earlier this year, with Charlotte and Louis showing off the 'trust' and 'affection' they share in each other. 

"Charlotte leans to listen properly to her brother, shortening the space between them, which shows that her caring nature is far beyond her eight years, and Louis talks to her while looking at her face, showing that he trusts her," body language expert Inbaal Honigman told The Express at the time.

“They both appear to feel safe, and their movements are soft and confident, not remotely jumpy or insecure.”