The British and Belgian Royal Families have been related since the birth of the modern Kingdom of Belgium and the reign of Queen Victoria in Britain when she married Prince Albert of Saxe-Coburg-Gotha. Queen Victoria was the niece of King Leopold I of the Belgians (Prince Albert was his nephew also) and the countries have been aligned (not formal allies exactly because of neutrality for many years) and today King Albert II and Queen Elizabeth II are third cousins. The Belgian royal house will be represented at the wedding of Prince William and Kate and certainly the Belgian family royal wishes their British cousins a very happy marriage together and join in happiness for them on that special day.
King George V and King Albert I in World War I
King George VI and King Leopold III between the wars.
Queen Elizabeth II and King Baudouin watching victory day celebrations at Buckingham Palace