She continued: “My sister makes my world so bright and I somehow accept when she says I am not right!
“Today is National Sibling Day and I raise a toast and am happy to boast that my sister is the finest in the land.
“So thank you my dearest Jane I stand and applause with both my hands ... Love you Jane,” the Duchess wrote, using the hashtags #nationalsiblingday and #sisters.
A follow-up snap showed Jane smiling while riding on a carousel at the carnival-themed fete, which was reportedly held the day after the nuptials.
Sarah’s gushing post comes after she recently took to Instagram to share another previously unseen throwback snap of her daughter Eugenie, which was taken on her big day.
In the delightful snap, which Sarah posted for her daughter’s birthday in March, Eugenie is all smiles as she poses for a close-up picture, tilting her head to one side.
Fans were also given a close-up of Eugenie's dazzling Greville Emerald Kokoshnik tiara and her Peter Pilotto and Christopher De Vos wedding dress.
"My favourite photograph just for her mum on her wedding day… and my little girl is 30 today and Happy Birthday my little magic dust,” Sarah captioned.
Princess Eugenie later shared some of her own pictures to mark the milestone occasion, including a photo from her christening day.
"Thank you for all the wonderful birthday messages,” she wrote.