Kate Middleton Established ‘Life Rules’ with Royals: ‘Her Priority Would Always…see more

Kate Middleton is loyal to the crown, but she’s also loyal to her own personal ethos, including the firm belief that her children Prince George, Princess Charlotte and Prince Louis come first.

Raised in a tight-knit family of five outside of the royal fold, biographer Robert Jobson writes in his new book Catherine, the Princess of Wales that Kate “knows her own mind and even established some fundamental life rules that she would stick to when she joined ‘The Firm.’ ” Kate presented her terms to Queen Elizabeth and the future King Charles in early 2015, as she was pregnant with Charlotte, “not in some formal document, but by Prince William,” Jobson says of Kate’s husband since 2011.

Just as Kate prioritizes her children, she also is firm on focusing on her health as she continues her treatment for cancer, a diagnosis she announced in March.

“She is doing things slowly and when she is ready,” Jobson told PEOPLE of Princess Kate’s return to public duty this summer, including appearances at Trooping the Colour in June and Wimbledon in July. “She isn’t being governed by it being a good picture opportunity. They aren’t worried about visibility — they are just going to do it, and that will be visible rather than the other way around. The health and the proper recovery is what is important.”

This summer, Jobson told PEOPLE that the Prince and Princess of Wales’ family of five will head out to their country home, Anmer Hall: “They escape there, and it is very much a private place,” he said. “They can go out and ride their bikes with the children and not have anybody bothering them. That will be the priority.”

He added, “They can go out and enjoy the space and can breathe out.”

Jobson said he also expected the fivesome to head to the royal family’s traditional August getaway spot, Balmoral Castle in Scotland, “but not for very long,” he told PEOPLE. “Those traditions may fade— that’s where you may find that it is more for the King [Charles] and the Queen [Camilla] and the other members of the royal family. William has more of a free rein. They will go there, but it won’t necessarily be for a long period like it used to be with the late Queen [Elizabeth].”

As Princess Kate continues to receive cancer treatment, she and Prince William “are living in a day-to-day basis, because no one really knows what’s around the corner,” Jobson told PEOPLE. Kate’s diagnosis earlier this year “shows that you can’t take anything for granted,” he added. “Both the Prince and Princess of Wales are acutely aware of that, and that’s why they are spending as much time as [they] can with their children, whilst they can. One day, they may not be able to do so—certainly William. Both are acutely aware of that.”

The couple’s biggest challenge, Jobson said, “is to keep family life going while preparing George for his role without freaking out neither him nor Charlotte as to what that role is. It is quite a delicate balance. It’s a hard job to do that.”