Pippa Middleton’s friends have lifted the lid on her relationship with Princess Kate.

The Princess of Wales, 42, announced last week she is undergoing a course of preventative chemotherapy for an undisclosed form of cancer.

James Middleton, Kate’s younger brother, took to Instagram almost immediately to post an unseen childhood picture of himself and the princess alongside a heartfelt message.

He wrote: “Over the years, we have climbed many mountains together.

Pippa, 40, does not have a social media account, however, her close friends have shed light on the relationship between herself and Kate.

A source close to Pippa said: “The three siblings are incredibly tight … Pippa not only as a sister but as a best friend she can confide in.”

They continued to tell The Times: “They are extremely loyal to one another.”

James Middleton and Pippa Middleton attending King Charles’s Coronation

One year ago, the Middleton sisters embarked on a ski trip with their families in the snow-capped mountains of

They managed to keep their holiday secret for a few days of fun with their husbands and young children.

Both Pippa and Kate have three children each of their own around a similar age.

The Princess of Wales tied the knot with Prince William in 2011, and together they share Prince George, 10, Princess Charlotte, eight, and Prince Louis, five.

James Matthews and Pippa Middleton tied the knot in 2017

Kate Middleton and Pippa Middleton are close friends, sources claim

Meanwhile, Pippa married hedge fund manager James Matthews in 2017, and are parents to Arthur, five, Grace, three, and Rose, one.

Kate and her younger sister used to live together in a Chelsea flat during their 20s, a sign of their close relationship.

The 40-year-old was also a bridesmaid during the Prince and Princess of Wales’s wedding on the world stage.

Pippa now lives a stone’s throw away from the princess’s home in Adelaide Cottage on the Windsor Estate.