“They are in this together, Kate and the King,” royal biographer Sally Bedell Smith tells PEOPLE

Kate Middleton and King Charles’ bond might grow stronger as they both navigate cancer during the first time in the monarchy’s history that two senior royals are facing the diagnosis simultaneously.

“He has always had a very good bond with her. I don’t think it is presumptuous to say that she is like the daughter he never had,” royal biographer Sally Bedell Smith tells PEOPLE exclusively in this week’s cover story. “He shares with William an impulse to protect her. They are in this together, Kate and the King.”

Buckingham Palace said that King Charles, 75, was “so proud of Catherine for her courage in speaking as she did” to announce her cancer news on March 22.

The day before Princess Kate’s vulnerable video sent shockwaves around the world, he and Kate shared a private lunch at Windsor Castle.

Apart from being King and future queen, they are also “two patients going through a common health experience,” says a royal source, noting they “are bound to have a close connection… I imagine there was some comfort [for Kate] in seeing… that it was possible to balance some private information without there being an imperative to share all.”

The news from the Princess of Wales, 42, followed weeks of intense social media speculation about her health and whereabouts following abdominal surgery in January. The frenzy reached a fever pitch with the release of a photo shared by the palace for U.K. Mother’s Day on March 10, in which Kate was shown looking vibrant alongside her three children Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5.

When major news agencies pulled the photo after determining it had been manipulated, Kate admitted to “editing” the image and apologized.

The rare misstep only served to fuel the rumor mill. Then came a distressing report on March 19 that staff at the London Clinic (where the Princess of Wales had abdominal surgery) allegedly attempted to access her medical records. It quickly became clear that the Prince and Princess of Wales were at the center of a storm unlike any other they’d faced before.

“It unleashed a pent-up feeling that people wanted information — unfortunately, that’s the curse of being a modern royal,” says a former longtime palace aide.

In stark contrast to King Charles’ swift and clear cancer announcement on Feb. 5, Princess Kate’s health news came after weeks of uncertainty and confusion. Royal historian Amanda Foreman says of the chaos, “This was the day that the royal family came crashing down to earth, and you realize they are just human beings.”

Amid the challenges of his wife and father both having cancer — and the strain that remains with his brother, Prince Harry — Prince William, 41, is focused on his family.

“There will be a lot more pressure on William, but he’s always been aware of that. He has evolved to lead, and he is leading,” says the former palace aide.