Article continues below advertisementA red line is once again defining Prince Harry's relationship with the royal family, as hopes of a long-awaited reunion with King Charles collide with a familiar sticking point: security.The Duke of Sussex is reportedly eager to bring Meghan Markle and their two children, Prince Archie and Princess Lilibet, to the U.K. this summer - potentially for a visit to Sandringham - but only under specific conditions. After nearly four years without an in-person meeting between the King and his grandchildren, the possibility of a reunion is drawing attention.