United Kingdom, July 6 -- James Middleton is "incredibly proud" of his "darling sister", Catherine, Princess of Wales following her Three Peaks Challenge.

The 44-year-old royal scaled Scotland's Ben Nevis (1,345m), England's Scafell Pike (978m), and Wales' Mount Snowdon (1,085m) solo within 24 hours last weekend, and her brother has reflected on the conversations they had when she was receiving treatment for cancer two years ago.

Sharing a photo of him and Catherine hugging, as well as one of them with their parents, Michael and Carole Middleton, the princess' husband Prince William and their three children, which were both taken after she completed the challenge, James wrote on Instagram: "I'm so incredibly proud of my darling sister ....