The royal mum, 30, posted a previously unseen photo from Princess Beatrice and Edoardo Mapelli Mozzi's wedding day in Windsor last July.
Beatrice wearing the Queen's vintage Norman Hartnell dress and Queen Mary's diamond fringe tiara beams at her sister behind the camera, as her new husband grins by her side.
WATCH: Princess Beatrice and Edoardo Mapelli Mozzi announce pregnancy
A second image appears to show the newlyweds on holiday in front of a waterfall.
Eugenie – who welcomed her first child, son August, in February – also added a sweet message to her pregnant sibling on Instagram.
Beatrice is expecting her first child with husband Edoardo Mapelli Mozzi later this year.
Eugenie shared a previously unseen photo from her sister's wedding day
Alongside the snaps, Eugenie wrote: "It's World Bee Day!! A day to celebrate nature's most badass little friend with their black and yellow stripes...
"But our furry little friends have to share today with my big sis Bea, who's got a little one on the way.
"Beabea and Edo huge congratulations on your news - can't wait to meet the little one."
The second snap of Beatrice and Edoardo shared by Eugenie
Buckingham Palace announced the news of Beatrice's pregnancy on Wednesday, sharing a statement which read: "Her Royal Highness Princess Beatrice and Mr Edoardo Mapelli Mozzi are very pleased to announce that they are expecting a baby in autumn of this year.
"The Queen has been informed and both families are delighted with the news."
The Princess tied the knot with property developer Edoardo at the Royal Chapel of All Saints at Royal Lodge Windsor in July 2020. Due to coronavirus restrictions at the time, the private ceremony was attended by the couple's closest friends and family, including the Queen and the late Duke of Edinburgh.
Beatrice and Edoardo are expecting their first child this autumn
While the baby will be Beatrice's first child, Edoardo already has a son Christopher Woolf (affectionately known as Wolfie), from his previous relationship with American architect Dara Huang.
Ninth in line to the throne, Beatrice will move to tenth place when the Duke and Duchess of Sussex's baby arrives.
The princess's son or daughter will be 11th in line to the throne when he or she is born, which means that Beatrice's younger sister Princess Eugenie will move down to 12th place.
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