James Middleton has revealed he joined the Princess of Wales for parts of the Three Peaks Challenge to honour the promise he made to his 'darling sister' after her cancer diagnosis in an emotional message. After a video showing James helping his sister down Mount Snowdon was posted online, the youngest Middleton sibling, 39, announced they completed a portion of the gruelling endurance event together in a post on Instagram last night. James, who runs dog food brand Ella & Co, shared a moving snap of the future Queen hugging him, taken by royal photographer Matt Porteous, with the caption: 'I’m so incredibly proud of my darling sister.'Two years ago, I told you we'd climb this mountain together,' he wrote. 'We talked about climbing mountains while you were in hospital, and about the incredible healing power that nature can have for both the body and the mind.' James said it was an 'honour' to join Catherine for the physical challenge, which involves scaling three of the highest peaks in the UK within 24 hours, to spread her message about life after cancer and raise funds for the Royal Marsden Cancer Charity. He praised his older sister's 'strength, resilience, and determination' to complete the Three Peaks Challenge 18 months after announcing she had completed her chemotherapy treatment and is in remission from the disease. 'Your strength, resilience, and determination, all while being the most amazing mother, wife, daughter, and sister, inspire me and so many others every single day,' James wrote. 'Keep being exactly who you are,' his beautiful message to Catherine read. James Middleton has revealed he joined the Princess of Wales for parts of the Three Peaks Challenge to honour the promise he made to his 'darling sister' after her cancer diagnosisLast week, footage of James sweetly escorting his sister down to the base of Mount Snowdon in Wales, fuelled speculation that he undertook some part of the challenge with Catherine. The clip filmed by fellow hiker, entrepreneur Phillip Tebajjwa, showed Catherine, wearing red shorts, hiking boots, and a grey cap, holding James's hand as her famous sapphire engagement ring caught the light. The moment felt especially poignant as royal fans recalled how Catherine attended therapy appointments with James when he was being treated for severe depression that left him contemplating suicide.Speaking to Mail columnist Bryony Gordon, the youngest Middleton sibling said that while his therapist encouraged him to bring his family to his sessions, he only felt comfortable taking his older sisters, Catherine and Pippa.James described his sisters' engagement in his recovery journey as a 'very moving moment' - much like the author's presence alongside Catherine at Mount Snowdon over the weekend, X users said.'James has truly always been there for her like a fierce and proud brother,' one person wrote, while another added it was 'lovely' to see the bond between the siblings.It comes after James shared his support for the future Queen on social media, writing he was 'so proud' under Catherine's Instagram clip about her experience. Catherine's proud parents, Carole (far left) and Michael Middleton, also travelled to Wales to support their daughter, as seen in a family portrait taken by royal photographer Matt Porteous. James Middleton can be seen helping his sister, the Princess of Wales , down to the base of Mount Snowdon in a heartwarming video filmed by a hiker who also completed the Three Peaks Challenge After Catherine revealed her cancer diagnosis in March 2024, James shared a touching message of support alongside a childhood photo of the pair Further reacting to a photo of the Princess beaming with delight after completing the challenge, James discreetly left two emojis - a mountain and a heart - under the portrait.It appeared to be a reminder of the royal's own strength, over two years after Catherine announced she was undergoing chemotherapy treatment after undergoing planned abdominal surgery.At the time, James - who is an avid hiker himself - shared a touching message of support for Catherine on his Instagram profile.Alongside a childhood photo of the pair during a hiking trip, he wrote: 'Over the years, we have climbed many mountains together. As a family, we will climb this one with you too.'And her family - including her three children - was certainly by her side when Catherine reached the finish line, as seen in gorgeous new portraits released by the Princess of Wales last evening. Catherine's proud parents, Carole and Michael Middleton, also travelled to Wales to support their daughter, as seen in a family portrait taken by royal photographer Matt Porteous.In one touching image, the princess could be seen warmly embracing her daughter, who snuggled up close to her while she leaned her cheek against her head.In another, Charlotte reached towards her mother's hand – meanwhile, the royal's youngest child, Louis, was captured in the background playing with a dog belonging to James.Catherine captioned the images: 'This time last week, completing the National Three Peaks Challenge. A huge thank you to everybody who has supported The Royal Marsden Cancer Charity.'