Princess Diana Was the 'First Person to Rattle the Cage of the Monarchy,' Says Queen's Biographer

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Princess Diana left a huge impression on the royal family, including Queen Elizabeth.

"Diana was the first person to rattle the cage of the monarchy within living memory," the Queen's biographer Ingrid Seward tells PEOPLE in this week's issue, which looks back at Diana's legacy 25 years after her tragic death on Aug. 31, 1997.

"I don't think Diana went out intentionally to change the family," Seward, who knew Diana personally, adds. "She did it because that was the way she was."

RELATED: 12 Ways Princess Diana Broke with Tradition and Paved a New Path for the Royal Family

getty Princess Diana, Prince Charles and Queen Elizabeth

The Queen was criticized for staying silent in the days that followed Princess Diana's death.

"It felt pretty perilous," says Ed Perkins, director of the new HBO documentary The Princess.

On the evening of Sept. 5, the monarch finally addressed the nation — as both "your Queen" and "as a grandmother" — in a rare televised speech. Despite her usual stoic nature, Queen Elizabeth appeared to realize that she needed to express emotion...following Princess Diana's lead.

In recent years, the 96-year-old Queen has shown her ability to express warmth, even participating in comedy skits like at the 2012 Olympics with Daniel Craig as James Bond and sharing the screen with Paddington Bear for her Platinum Jubilee earlier this summer.

"Who knows whether an event like that could have happened without someone like Diana coming in?" says Perkins.

Adds Seward: "The Queen can now see Diana's legacy is a huge force for good."

PEOPLE presents The Story of Diana: Archival footage and conversations with those who knew the Princess of Wales best reveal how her story remains relevant today. Now on PeopleTV.