The Princess of Wales is close to her family, the Middletons, but she has a particularly special bond with her younger sister, Pippa Middleton.
The author and columnist was spotted looking gorgeous in a red ruffled dress as she attended a wedding with her husband, hedge fund manager, James Matthews, on Lake Como in Italy at the weekend.
It comes ahead of Pippa's 40th birthday on Wednesday 6 September, and no doubt Kate, Prince William and their children, will reunite with the Middletons to mark the occasion.
Pippa and her husband James also have three children – Arthur, Grace and Rose – who are close in age to their royal cousins, Prince George, Princess Charlotte and Prince Louis.
The sisters have supported one another through some of their biggest life events, including Pippa's starring role at William and Kate's royal wedding, which you can watch below.
While Pippa and James have reportedly swapped their £17million Chelsea pad for a quieter life in the Berkshire countryside to be closer to Pippa's parents, Carole and Michael, and younger brother, James Middleton, she and Kate navigated their twenties together before Kate married her Prince in 2011.
HELLO! takes a closer look at their sisterly bond…
Kate and Pippa's childhood
The Middleton sisters, born just 21 months apart, grew up in Bucklebury, Berkshire and both attended boarding school, Marlborough College.
Kate and Pippa shared a love of sport, including hockey, tennis and athletics, which has followed them into adulthood.
As their mother Carole's company, Party Pieces, went from strength to strength, her children helped out at the company.
In an interview with The Telegraph in 2018, Carole revealed: "They did a lot of modelling. Catherine was on the cover of one of the catalogues, blowing out candles. Later on, she did some styling and set up the First Birthday side of the business. Pippa did the blog. I still value their ideas and opinions."
Before Kate married Prince William
After graduating from university, Kate and Pippa shared a £1.88million apartment in Chelsea, close to King's Road.
While future royal Kate worked as an accessory buyer for clothing brand, Jigsaw, Pippa had jobs in public relations and event management.
The Middleton sisters were often spotted on nights out together at popular hotspots, including Mahiki and Boujis.
In an interview with NBC in 2014, Pippa said of her relationship with her big sister: "I mean, obviously she has pressures that she's taken on and things. But we spend a lot of time together. We still do a lot together as a family. We have a very normal, sisterly relationship. We're very close. And, you know, we support each other and get each other's opinions and things."
Royal wedding 2011
Pippa had a huge supporting role as Kate's maid of honour at the royal wedding in April 2011. While she might have made headlines for her stunning bridesmaid dress, Pippa was there to hold the train of Kate's Alexander McQueen gown as well as lead the young bridal party into Westminster Abbey.
Speaking about the big day to NBC, Pippa said: "It sounds funny to say, but we saw it as just a family wedding. And actually, I didn't realise, perhaps, the scale of it until afterwards. We all took on the roles as any family would."
Pippa Middleton's wedding 2017
Kate repaid the favour to her sister in a supporting role at Pippa's nuptials in Englefield in May 2017. Looking chic in a pale pink McQueen dress, the then Duchess of Cambridge kept a watchful eye on the young bridal party, including her own children, Prince George and Princess Charlotte.
The royal was even spotted adjusting Pippa's veil before she walked down the aisle.
Royal christenings
Pippa is a devoted auntie to Kate's three children, having attended the royal christenings of Prince George, Princess Charlotte and Prince Louis.
Kate and Pippa's love of sport
The Princess has showcased her sporty skills through her royal engagements, while Pippa has taken on many gruelling challenges, including marathons and a bike race.
The sisters have also been long-time tennis fans, with Pippa joining her sister in the royal box at Wimbledon in 2012 and 2019, alongside the Duchess of Sussex.
Pippa supporting Kate's royal projects
The Middletons have turned out to support Kate's Christmas carol concert two years in a row.