The news is a welcome boost for Sarah, 63, just days after she spoke at the funeral of her friend Lisa-Marie Presley.
"You will be sharing puddles, Augie ! Superb news, Granny heaven…. So deeply grateful," she captioned the post.
Sarah shared an adorable shot of her grandson August
Her followers were quick to comment on the news, with one writing: "Congratulations, Sarah. Such wonderful news for you all!"
Another wrote: "Congratulations, Granny!" while a third said: "Over the moon for you all! Xxx."
It comes after Princess Eugenie broke the happy news on her own Instagram account, sharing a sweet family photograph of her son August kissing her bump.
Eugenie wrote: "We’re so excited to share that there will be a new addition to our family this summer."
A Buckingham Palace spokesperson said: "Princess Eugenie and Mr Jack Brooksbank are pleased to announce they are expecting their second child this summer.
"The family are delighted and August is very much looking forward to being a big brother."
August turns two on 9 February
The tot will be 13th in line to the throne and will be a plain Miss or Master.
Meanwhile, big brother August will celebrate his second birthday on 9 February.
Eugenie's sister Princess Beatrice is also a mother of one, and her daughter Sienna, who she shares with Edoardo Mapelli Mozzi, was born just seven months after August, in September 2021.
The sisters don't always act in such harmony, however. In this video you can see the ways in which they are different.
WATCH: Beatrice and Eugenie differences
Listen: The Harry and Meghan A Right Royal Podcast special
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