The royal brothers -- sons of the late, great Princess Di -- granted a rare interview to Katie as the Queen's Diamond Jubilee events approach.
"Behind closed doors, she's our grandmother, it's as simple as that," Harry said.
"As I learned from growing up, you don't mess with your grandmother," said William as he walked with Katie around the breathtaking grounds of Buckingham Palace.
Watch the video on RadarOnline.com
The chat is a huge coup for Katie, who launches her own self-titled talk show September 10.
The Real Queen By Her Own Royal Family airs on ABC on May 29 at 9/8c.
Take a look at the sneak peek on RadarOnline.com below:
- 'The Damage has Already Been Done': Prince Harry Shouldn't Expect More Than a 'Text' From Prince William on His Birthday, Insider Claims
- 'Don't Hear From Him': Prince Harry Ghosted His Longtime Pals After Moving to America With Meghan, King Charles' Former Butler Claims
- Royal Olive Branch: Prince Harry and Meghan Hope to Repair Family Rift by Renting Apartment in Kensington Palace