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 Christine Louise of Oettingen-Oettingen Christine Louise of Oettingen-Oettingen (20 March 1671 in Oettingen – 3 September 1747 in Blankenberg) was Duchess of Brunswick-Lüneburg. She was the maternal grandmother of Holy Roman Empress Maria Theresa, Queen Elisabeth Christine of Prussia, Queen Juliane Marie of Denmark and Norway and Tsar Peter II of Russia; the maternal great-grandmother of Queen Marie Antoinette of France, Tsar Ivan VI of Russia and Francis, Duke of Saxe-Coburg-Saalfeld; the maternal great-great-grandmother of King Leopold I of Belgium and the maternal great-great-great-grandmother of Queen Victoria of the United Kingdom.