Princess Kate's 'Rock': Pippa Middleton Remains Close to Her Sick Sister During Future Queen's Cancer Battle

Princess Kate's younger sister, Pippa Middleton, has been called the Princess of Wales' "rock," in the wake of the future queen's cancer diagnosis.

"We know that she's got a very close and supported family, and secondly, that the support is under the radar," royal expert Richard Fitzwilliams told an outlet when discussing Kate's illness.

"The Middletons are clearly a very, very important part of her and William's life," he added. "At the moment absolutely pivotal. I would say. We know that there is a tremendously strong bond between her mother in her parents, also siblings, and also that they live in close proximity or relatively close proximity. So yes, absolutely, I think that this is important. I think the Middletons will be her rock, and they will be also tremendously helpful, and especially regarding the children."

This insight comes amid the shocking news that the future queen is sick with a potentially terminal illness.

The Princess of Wales announced she was battling cancer with chemotherapy in a video message released on Friday, March 22. The future queen did not reveal the type of cancer she has.

"I wanted to take this opportunity to say thank you, personally, for all the wonderful messages of support and for your understanding whilst I have been recovering from surgery," the princess shared. "It has been an incredibly tough couple of months for our entire family, but I’ve had a fantastic medical team who have taken great care of me, for which I am so grateful."

At the end of the message, the future queen movingly reminded "all those suffering with this disease" to "never lose hope" and reminded them that "they are not alone."

"As you can imagine, this has taken time," the queen-in-waiting continued. "It has taken me time to recover from major surgery in order to start my treatment. But, most importantly, it has taken us time to explain everything to George, Charlotte and Louis in a way that is appropriate for them, and to reassure them that I am going to be ok."

"As I have said to them; I am well and getting stronger every day by focusing on the things that will help me heal; in my mind, body and spirits," the senior royal added. "Having William by my side is a great source of comfort and reassurance too. As is the love, support and kindness that has been shown by so many of you. It means so much to us both."

The disease is said to be "in its earliest stages," which is also where Kate's treatment currently is. "My medical team therefore advised that I should undergo a course of preventative chemotherapy, and I’m now in the early stages of that treatment," HRH explained. "This, of course, came as a huge shock, and William and I have been doing everything we can to process and manage this privately for the sake of our young family."

The Sun interviewed Fitzwilliams.