Sarah Ferguson on How 'Proud' Princess Diana Would Be of Her Sons and Their 'Wonderful Wives'

Sarah "Fergie" Ferguson grew up alongside Princess Diana — and today she says her childhood friend would be brimming with pride at the men her sons have become, along with the women who are their partners.

"She would be very proud of her sons and their wives," the Duchess of York, 61, tells PEOPLE exclusively in this week's cover story. "And she would be just like me, obsessed with her grandchildren. Because that's what she loved." Close with the Duchess' daughters Princess Eugenie, 31, and Princess Beatrice, 32, Diana "adored my girls. She adored the boys. This would be her haven. Her heaven."

"If she were sitting with me right now, I know she would say, 'I am so proud of both of my boys and the wonderful wives they have chosen,'" she says of Diana's sons Prince William, 39, and Prince Harry, 36, along with Kate Middleton, 39, and Meghan Markle, 39. "Because each has got her own voice."

David Burton Sarah Ferguson, Duchess of York on the cover of PEOPLE.

Watch the full episode of People Cover Story: Sarah Ferguson on PeopleTV.com or on the PeopleTV app.

The Duchess, whose debut novel, the sweeping coming-of-age story Her Heart for a Compass, will be published on August 3, first met the then-Diana Spencer when they were teenagers.

Sarah Ferguson's new novel, Her Heart for a Compass.

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"We were best friends from when she was 14 and I was 15," says the Duchess, who affectionately called Diana "Duch."

Mike Forster/Daily Mail/Shutterstock Princes Diana and Sarah, the Duchess of York in 1991 with Prince William (left) and Prince Harry (center).

RELATED: Sarah Ferguson Celebrates Daughter Princess Eugenie's Baby Boy: 'I'm a Granny!'

"She taught me so much of public life," says the Duchess. "She was so brave. We used to have the most incredible time together."

For the full exclusive interview with Sarah Ferguson, Duchess of York, pick up this week's issue of PEOPLE, on newsstands Friday

Georges De Keerie/Getty Sarah Ferguson and Princess Diana in 1983.

Of past reports that she and Diana fell out before Diana's 1997 death, the Duchess says, "We promised each other we would always be together — there was never any daylight between us. But everybody wanted that, because we were so strong together. People want to break something so strong."

Listen below to our daily podcast PEOPLE Every Day for more on PEOPLE's cover story interview with Sarah 'Fergie' Ferguson.