But for the first time ever, the brothers are coming together in a new documentary to discuss Diana’s parenting style and the many ways her influence had shaped their lives.
In a promo for Diana, Our Mother: Her Life and Legacy — the documentary commissioned by the U.K.’s ITV network that will air in the States on HBO — William and Harry look through a private family album together that was assembled by their mother, recalling some of their most touching and joyful memories from their childhood.
“This is the first time that the two of us have ever spoken about her as a mother,” Harry, 32, says — as the two study a photo of a young Diana and William together.
“Believe it or not, you and I are both in this photograph,” William, 35, tells his brother. “You’re in the tummy!”
The brothers gave insight into Diana parenting style in a way she’s never been seen before — with Harry calling her “one of the naughtiest parents.”
“She was very informal and really enjoyed the laughter and the fun,” William says. “She understood that there was a real life outside of Palace walls.”
Even in death, Harry says Diana’s love still carries on. “She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world,” he says. “She smothered us with love, that’s for sure.”
Diana died at the age of 36 following a car crash in Paris on August 31, 1997. William and Harry were just 15 and 12 at the time.
As the 20th anniversary of her death nears, the boys are participating in many tributes for their mother — including unveiling a public commemoration of their mother: a statue that will be erected in her honor at Kensington Palace.
RELATED VIDEO: William and Harry Open Up About Diana Together for the First Time
The boys also filmed a BBC documentary called Diana which has yet to be released, in which William admitted they feel that they “owe” it to their mother to “stand up for her name and remind everybody of the character and person that she was.”
“I think an element of it is feeling like we let her down when we were younger,” William said in a preview for in the 90-minute film. “We couldn’t protect her.”
“I think it’s never going to be easy for the two of us to talk about our mother, but 20 years on seems like a good time to remind people of the difference that she made not just to the royal family but also to the world,” Harry added.
PEOPLE has also teamed up with ABC for a television celebration of Diana’s life and legacy titled The Story of Diana. Set to premiere over two nights in August, the four-hour special will offer fresh perspective on the woman known around the world as the People’s Princess.
Her brother Charles, 9th Earl Spencer will lead new interviews from those who knew her best, including friends Sir Richard Branson, Dr. James Colthurst, Vivienne Parry and Wayne Sleep as well as former palace staffers Mervyn Wycherley and Colin Tebbutt. Her wedding dress designer Elizabeth Emanuel and bridesmaid India Hicks also contribute.
Jerry White, who traveled with Diana on one of her groundbreaking anti-landmine missions, and Tessy Ojo, CEO of the Diana Award, will underline how her legacy continues to this day, while Jess Cagle, Editor-in-Chief of PEOPLE, and ABC’s Elizabeth Vargas will talk about the woman who was so loved around the world.
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“One of the reasons I wanted to talk now is because I think after 20 years, somebody shifts from being a contemporary person to one of history, actually,” Spencer explained in a clip released June 29. “And Diana deserves a place in history. This was a special person, and not just a beautiful one.”
Diana, Our Mother: Her Life and Legacy premieres Saturday on ITV and HBO later this month.
The Story of Diana: Part One airs on August 9 at 9 p.m. ET and The Story of Diana: Part Two on August 10 at 9 p.m. ET — both on ABC.