Catherine, Princess of Wales is set to join the royal family at Balmoral this month. The 42-year-old royal will accompany her husband Prince William and their three children, Prince George, 11, Princess Charlotte, nine, and six-year-old Prince Louis, as well as other members of the institution, to Scotland for their annual break, in what will be her longest journey since she was diagnosed with cancer earlier this year, sources told the Mail on Sunday newspaper. After revealing in March she was undergoing preventative chemotherapy following abdominal surgery earlier in the year, Catherine has m…