Kate's nephew, Arthur, who is the eldest child of Pippa and her husband James Matthews, is turning five on Sunday. So it wouldn't be a surprise if Kate, Prince William and their kids Prince George, Princess Charlotte and Prince Louis all met up with Pippa and her family to celebrate Arthur's big day in person.
It's also the start of the holidays for George, Charlotte and Louis as Lambrook breaks up for its two-week October half-term on Friday, so a family birthday party would be one wonderful way to kick off the school break.
Kate and Pippa, who are just 21 months apart, are incredibly close and have remained tight even after the royals left London. They were previously neighbours when William and Kate lived in Kensington Palace and Pippa and James owned a £22.5million mansion in Chelsea, west London.
The two families are about a 40-minute drive away from each other and given the privacy of their country estates, it's far easier to meet up away from the public glare.
Pippa is also around the corner from her parents Carole and Michael Middleton, who live in their £5million Bucklebury Manor, and is said to have been motivated to move to Berkshire to be closer to her family.
A big family reunion is perhaps on the card for little Arthur's fifth birthday. Pippa and James welcomed their eldest child on 15 October 2018 at the Lindo Wing of St Mary's Hospital in London – the same hospital where Kate gave birth to her three children.
Arthur's birth made William and Kate an uncle and aunt, and at the time, the couple released an official statement saying they were "thrilled" for the new family.
Arthur's full name is Arthur Michael William Matthews; Michael pays tribute to Pippa's dad, as well as James Matthews' late brother Michael, who sadly died at the age of 22 climbing Mount Everest in 1999.
Pippa and James have gone on to welcome two daughters – Grace Elizabeth Jane, who is two and a half, and Rose Louise Victoria, one.
The mother-of-three has previously spoken about her close sisterly bond with Kate, telling NBC in 2014: "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."
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