The Princess of Wales’ brother James Middleton broke his silence on Kate’s cancer diagnosis, RadarOnline.com has learned. After Kate, 42, revealed in a video message on Friday afternoon that she has been diagnosed with cancer, James, 36, posted a heartfelt message to his sister online. James took to Instagram shortly to share a sweet message after news of his sister’s health made headlines. The 36-year-old shared a throwback photo of him and Kate along with the caption, “Over the years, we have climbed many mountains together. As a family, we will climb this one with you too.” Support for Kate…