Sarah, Duchess of York still feels Princess Diana's presence "every single day".
The 62-year-old royal was "best friends" with the late Princess - who died in Paris in August 1997, aged 36 - has opened up about the bond they shared, and she described her as "the best person" she's ever met.
Speaking to Italy's Oggi magazine, Sarah said: "Diana had two sons and I had two daughters. Yes we used to fantasise about the future, we wondered how it would be and it was fun.
"If today our children are friends that's because they have it in their blood, it's like we passed them what we used to feel, our bond, because we were best friends...
"It's very, very sad she is not among us anymore. Although I feel her presence very close to me every single day. I think she is the best person I've ever met."
Sarah - who has daughters Beatrice and Eugenie with her ex-husband Prince Andrew - also opened up on her love for her girls admitted nothing can compare.
She added: "It's impossible for me to love my nephews more than my two daughters.
"I love Sienna and August, they know their grandmother loves them dearly, every time I see them, I know I'll make them laugh.
"But nothing is like the joy I feel looking at my daughters Beatrice and Eugenie trying to be good mother."
Meanwhile, the Duchess recently revealed she believes Diana would be delighted for her youngest son Prince Harry, who has moved from the UK to California with his wife Meghan, Duchess of Sussex and their children Archie, two, and six-month-old Lilibet.
She said: "The most important thing - and I know Diana, if she were here, would say that - is they are happy. And she makes him happy.
"And I love to see that little boy that cried at the funeral to have happiness now."