The Princess of Wales had a special family occasion to celebrate on Wednesday after stepping out at the Commonwealth Day service with husband, Prince William, on Monday.
Kate, 41, shares a close bond with her younger siblings, Pippa Matthews, 39, and James Middleton, 35, and is a doting aunt to Pippa's three children.
WATCH: Princess Kate wows in elegant peplum outfit at Commonwealth Day service
Pippa and her husband, James Matthews, marked their second child Grace's second birthday on 15 March, and no doubt they will have made a big fuss of the birthday girl.
At the time of her birth in 2021, a family source told HELLO!: "Mother and baby are doing well. She's perfect, everyone is overjoyed at such a happy arrival."
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Pippa and husband James share three children
The couple, who recently enjoyed a family holiday in St Barts are also parents to Arthur, four, and baby Rose, who was born last summer.
Kate and Pippa shared a flat together in London after graduating from university and had starring roles at each other's respective weddings in 2011 and 2017.
The Princess's eldest children, Prince George, now nine, and Princess Charlotte, now seven, were also page boy and flower girl for their aunt Pippa at her nuptials.
Pippa was Kate's maid of honour on her wedding day
Meanwhile, Kate's youngest child, Prince Louis, is just six months older than his cousin, Arthur.
Kate and Pippa, born just 21 months apart, grew up in Bucklebury, Berkshire and both attended boarding school, Marlborough College.
The Middleton sisters shared a love of sport, including hockey, tennis and athletics, which has followed them into adulthood.
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."
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