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.
Prince Christian of Schleswig-Holstein Prince Christian of Schleswig-Holstein (Frederick Christian Charles Augustus; 22 January 1831 – 28 October 1917) was a minor Danish-born German prince who became a member of the British Royal Family through his marriage to Princess Helena of the United Kingdom (25 May 1846 – 9 June 1923), the fifth child and third daughter of Queen Victoria and Prince Albert of Saxe-Coburg-Gotha.