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 Edward of Saxe-Weimar Prince William Augustus Edward of Saxe-Weimar-Eisenach (11 October 1823 – 16 November 1902) was a British military officer of German parents. After a career in the Grenadier Guards, he became Major General commanding the Brigade of Guards and General Officer Commanding the Home District in 1870, General Officer Commanding Southern District in October 1878 and Commander-in-Chief, Ireland in October 1885. He was promoted to field marshal in 1897 despite his career including no great military achievements.