Kate and William welcome their first child, Prince George, on 22 July 2013.
Speaking shortly after his birth, Sophie shared that she was excited with the news.
When asked how she felt, she replied: “I feel ridiculously happy actually, so excited for them both. It’s a wonderful event.”
Although the Countess was happy to hear about George’s birth, she explained she took a step back when asked if she had visited the newborn.
She added: “Not yet, I’ve sent messages, but I’m hoping in the next couple of days to be able to catch up with them.
“I think they’ll probably be wanting to just be together at the moment.”
Sophie has two children of her own with her husband, Prince Edward, 55.
The royal couple are the parents of Lady Louise Windsor, 16, and James, Viscount Severn, 12.
Sophie added: “Everybody is absolutely thrilled, as you can imagine. As I say, I feel ridiculously happy for them.
“I never expected to feel quite this happy but I’ve had a big smile on my face ever since the news broke.”
Since George was born, Kate and William have had two children, Princess Charlotte, who was born on 2 May 2015 and Prince Louis, born on 23 April 2018.
Sophie is thought to have a close relationship with the Queen and reportedly helped younger royals when they married into the family.
The Countess of Wessex is said to have watched out for royals including Kate Middleton and Meghan Markle when they entered the Royal Family.