Taking to her personal Instagram account, Eugenie posted a montage of photos including shots of the duo when they were children at a baseball game, swimming with dolphins and holding hands as they wore matching white dresses.
The snaps then progressed to more recent images of Beatrice, including pictures of her hugging her sister in a fond embrace.
Eugenie's sweet caption read: Happy Birthday to my big sissy... BeaBea you are sensational and I have loved every minute watching you be brave and true... and you [heart emoji]...and now a mummy to be :) #iloveyou." Aww!
WATCH: Beatrice and Eugenie's sweet sisterly bond
Eugenie is set to become an aunt when Beatrice gives birth to her first child this autumn, who will be the Queen's 12th great-grandchild.
It's been a special couple of years for the York family as Beatrice tied the knot with property developer Edoardo Mapelli Mozzi in a private Windsor ceremony last July.
Princess Eugenie shared the lovely tribute to Instagram
Princess Eugenie and her husband, Jack Brooksbank, also welcomed their first child in February 2021, naming their son August Philip Hawke Brooksbank.
The Princess regularly shares updates about her baby son on Instagram, including a video of him in an England sleepsuit during the Euro 2020 tournament and a photo of their trip to visit the Tea Timers, a herd of wooden elephants, which are part of the CoExistence Story from the Elephant Family.
Beatrice and Eugenie with their mother, Sarah, Duchess of York
Eugenie, 31, shares an incredibly close bond with her sister, Beatrice, with Eugenie proclaiming in an interview with The Telegraph in 2008 that: "Mummy, Bea and I call ourselves 'The Tripod', they are my best friends in the world."
They were both each other's maid of honours at their weddings and are regularly spotted out and about together at events.
Princess Beatrice Elizabeth Mary was born on 8 August 1988 at the Portland Hospital in London, but the Duke and Duchess of York did not announce her name until almost two weeks after her birth.
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