Arthur Gerald Wellesley, Earl of Mornington, is the great-great-grandson of what  last German Emperor ("Kaiser") and King of Prussia?
Wilhelm II, German Emperor Wilhelm II or William II (German: "Friedrich Wilhelm Viktor Albert von Preußen", English: Frederick William Victor Albert of Prussia; 27 January 18594 June 1941) was the last German Emperor ("Kaiser") and King of Prussia, ruling the German Empire and the Kingdom of Prussia from 15 June 1888 to 9 November 1918. He was the eldest grandchild of the British Queen Victoria and related to many monarchs and princes of Europe.
Princess Antonia, Duchess of Wellington Princess Antonia of Prussia, Duchess of Wellington, Duchess of Ciudad Rodrigo, Princess of Waterloo, Duchess of Victoria, (born 28 April 1955), is a great-granddaughter of Wilhelm II, German Emperor and the wife of Charles Wellesley, 9th Duke of Wellington.