The Princess of Wales' brother has sweet reaction to her announcement

James Middleton is a proud brother. The Princess of Wales ' youngest sibling reacted on Monday to the news that his big sister has completed her chemotherapy treatment.

"I couldn't be more proud ❤️," he commented on the video shared by Kensington Palace on Instagram.

After Her Royal Highness revealed in March that she was in the early stages of her chemotherapy treatment, James took to his personal Instagram to pen a touching message to his sister Catherine. Alongside a throwback photo of the two of them, James wrote: "Over the years, we have climbed many mountains together. As a family, we will climb this one with you too ⛰️❤️."

Less than six months later, the Princess of Wales announced in a video—released on Sept. 9—that she has finished her chemotherapy treatment . The moving video features Catherine, her husband Prince William , their three children, Prince George , Princess Charlotte and Prince Louis , and the Princess' parents, Carole and Michael Middleton .

HOLA! USA understands that the Princess of Wales has drawn strength from her family and loved ones as well as from spending time on things that bring her joy. Those elements and the sanctuary that she found in the natural world were all represented in the video, which was filmed last month in Norfolk.

In the video, the Princess said, "As the summer comes to an end, I cannot tell you what a relief it is to have finally completed my chemotherapy treatment. The last nine months have been incredibly tough for us as a family. Life as you know it can change in an instant and we have had to find a way to navigate the stormy waters and road unknown. The cancer journey is complex, scary and unpredictable for everyone, especially those closest to you."

"With humility, it also brings you face to face with your own vulnerabilities in a way you have never considered before, and with that, a new perspective on everything," she continued. "This time has above all reminded William and me to reflect and be grateful for the simple yet important things in life, which so many of us often take for granted. Of simply loving and being loved."

The Princess went on to share that her focus is now doing what she can to stay cancer free. "Although I have finished chemotherapy, my path to healing and full recovery is long and I must continue to take each day as it comes. I am however looking forward to being back at work and undertaking a few more public engagements in the coming months when I can. Despite all that has gone before I enter this new phase of recovery with a renewed sense of hope and appreciation of life," Catherine said. "William and I are so grateful for the support we have received and have drawn great strength from all those who are helping us at this time. Everyone’s kindness, empathy and compassion has been truly humbling. To all those who are continuing their own cancer journey – I remain with you, side by side, hand in hand. Out of darkness, can come light, so let that light shine bright."