The 64-year-old author hailed the news her daughter Princess Beatrice and her husband Edoardo Mapelli Mozzi - who has eight-year-old Wolfie from a previous relationship and three-year-old Sienna with his wife - are expecting another baby an "incredible blessing" and she's looking forward to seeing all the children, including Princess Eugenie's sons August, three, and Ernest, 15 months, together.
Sharing a series of family photos on Instagram, Sarah wrote: "A very proud GiGi/Mum today. Such an incredible blessing and an embrace to my heart. Wolfie, August, Ernie and Sienna will be over the moon to have the fifth member of the five aside! [purple heart emoji] (sic)"
She then addressed the rest of her message directly to Beatrice.
Sarah - who has her daughters with ex-husband Prince Andrew - wrote: "Darling Beatrice. Words can't quite express the joy and excitement I feel as you and Edo prepare to welcome another precious addition to your beautiful family".
"Becoming a granny again fills my heart with so much happiness and gratitude".
"Watching you grow as a mother has been one of the greatest privileges of my life, and now to see your family blossom further is an absolute blessing. Our family is growing, and so too is the love that surrounds us. I cannot wait to meet this little one and to share in all the wonderful memories we will make together. All my love."
Buckingham Palace announced the 36-year-old princess' pregnancy on Tuesday
A statement read: "Her Royal Highness Princess Beatrice and Mr Edoardo Mapelli Mozzi are very pleased to announce that they are expecting their second child together in early spring; a sibling for Wolfie, aged eight, and Sienna, aged three."
The statement also noted King Charles, Beatrice's uncle, has been informed and both families are "delighted" with the news.
The new baby will become 11th in line to the throne, just behind its older sister and mother but not be known by the title of HRH or have a royal title, as it is only the grandchildren of the reigning monarch who become known as prince or princess.