The Princess of Wales has an incredibly close relationship with her younger sister Pippa. They were born just 21 months apart, and are like best friends as well as siblings.
Both have gone on to welcome three children; Kate shares Prince George, ten, Princess Charlotte, eight, and Prince Louis, five, with Prince William, while Pippa shares Arthur, five, Grace, three, and one-year-old Rose with husband James Matthews.
The Middleton family are known to be very tight knit. Parents Carole and Michael play a hands-on role with their children and grandchildren. And Pippa and husband James are known to do the same, having been key in supporting the Princess after her abdominal surgery and cancer diagnosis. James Middleton, the Princess's brother, meanwhile, is also a pillar of support.
James was quick to publicly voice his love for his sister after her cancer diagnosis was made public. He posted a picture on Instagram showing them climbing in the Alps together as children, and captioned it, "Over the years, we have climbed many mountains together. As a family, we will climb this one with you too."
And Charlotte and Grace also already have something special in common. They both have Elizabeth as their middle name - a moniker passed on through the Middleton family via Carole and Kate who also share the sweet name. The Princess of Wales was born Catherine Elizabeth Middleton while her mother is Carole Elizabeth Middleton, née Goldsmith.
Meghan Markle 'to unleash her own memoirs' as Prince Harry's drops next weekOf course, for Charlotte, the name Elizabeth is also in honour of her paternal great-grandmother, the late Queen. Her second middle name, Diana, is a touching nod to William's late mother. Grace's full name, meanwhile, is Grace Elizabeth Jane Matthews - with Jane chosen as a nod to James's mother and Pippa's mother-in-law, Jane Matthews.
Certainly the Wales family and the Matthews family are close. According to the Times, the sisters jetted off for a skiing holiday together in Meribel last year, complete with their husbands and their children.
"They managed to keep their holiday secret for a few days of fun with their husbands and young children: cosy après-ski lunches together hidden in the snowy forest," the publication reported.
Royal expert Jennie Bond previously spoke to ! about Kate and Pippa, and reflected on their relationship behind closed doors. "They talk about everything and I very much doubt that Catherine expects her sister to stand on ceremony when they are together," Jennie said. "In public, of course, Pippa pays due respect to the future Queen, but I suspect they probably both find that sort of protocol quite funny."
As well as the Princess possibly wanting to not "stand on ceremony" behind closed doors, Jennie suggested that there's another reason Kate and Pippa's relationship has remained so "balanced" - and it's down to some similarities in their lifestyles and living arrangements.
"The fact that [Pippa] has married an extremely wealthy man, James Matthews, means that the relationship between the two sisters has remained quite a balanced one. Pippa and James have a riverside mansion in Berkshire, less than an hour's drive from Windsor (where Catherine lives) and half an hour from their parents," she explained.
The royal expert also expressed admiration for how Pippa has "kept her feet on the ground" despite facing the "blaze of publicity" that put her in the public eye when she was her sister's bridesmaid. And she added that Pippa is "by all accounts, a delightful young woman".