Prince William and Kate Middleton marked their 15th wedding anniversary on Wednesday with a new family photo . “Celebrating 15 years of marriage ❤️,” the Prince and Princess of Wales wrote via Instagram alongside a sweet snap with their children: Prince George, 12, Princess Charlotte, 10, and Prince Louis, 8. In the outdoor shot, William, 43, relaxed on the grass in a polo and shorts, two dogs on one side of him and his eldest child on the other. Prince William and Kate Middleton marked their 15th wedding anniversary on Wednesday. X / @KensingtonRoyal “Celebrating 15 years of marriage,” the Prince and Princess of Wales (pictured above at their April 2011 nuptials) wrote via Instagram. Getty Images Middleton, 44, lounged between George and Louis, holding hands with the little one. Charlotte was all smiles at the end of the group. William and his wife walked down the aisle at Westminster Abbey in April 2011, six months after their engagement .…