A friend tells PEOPLE that the Prince and Princess of Wales’ priority will be “to look after each other” during this time

Kate Middleton and Prince William visit Grange Pavilion as they celebrate the beginning of Black History Month on October 3, 2023 in Cardiff, Wales. PHOTO:

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Prince William is a pillar of strength for Kate Middleton as she receives cancer treatment.

“He is a very reliable, strong person. She doesn’t feel isolated in this because of William, who is right beside her,” a longtime family friend exclusively tells PEOPLE.

The Princess of Wales, 42, said in her video message announcing her health news that her husband of nearly 13 years is “a great source of comfort and reassurance.”

“This is about supporting her and her feeling supported. It is a joint effort. She doesn’t feel isolated at all,” the insider says. “The nuclear family that they have created so well is an immense support not only to him but to Catherine too.”

Kate’s parents, Carole and Michael Middleton, have been an additional source of help, with Carole accompanying her daughter on the children’s school drop-offs. Meanwhile, the royal’s siblings, Pippa and James also live nearby. James shared a touching statement on Instagram accompanied by a throwback photo of himself and Kate as children when Kate shared her news, writing, “Over the years, we have climbed many mountains together. As a family,we will climb this one with you too.”

Princess Charlotte, Prince William, Prince George, Mia Tindall, Prince Louis and Kate Middleton at Christmas 2023.

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While Princess Kate deliberately chose to film the personal video message about her health news alone, Prince William has been there for her every step of the way.

“It is a particular time in their lives, and I know their priority will be to look after each other,” the family friend explains. “His priority is on his family, and the other questions can wait.”

The Prince of Wales, 41, doesn’t plan to resume royal work until after Prince George, Princess Charlotte and Prince Louis’ school break is over later in April, aligning with a longtime philosophy of keeping duties to a minimum for more family time during school vacations.

“The children are at the center of their world,” a close friend says of the couple in this week’s PEOPLE cover story.

Prince George, Kate Middleton, Prince Louis, Prince William and Princess Charlotte on on the Buckingham Palace balcony during Trooping the Colour on June 17, 2023.

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Princess Kate referred to her young family several times in the address that a palace source confirmed she wrote herself, outlining how it took time to “explain everything” to George, 10, Charlotte, 8, and Louis, 5, “and to reassure them that I am going to be okay.”

“We hope that you will understand that, as a family, we now need some time, space and privacy while I complete my treatment,” the Princess of Wales said later in the message, which was recorded as she sat alone in the garden of Windsor Castle. “My work has always brought me a deep sense of joy, and I look forward to being back when I am able, but for now I must focus on making a full recovery.”