Who were the three rulers during World War 1 that were the head of their nations and were all cousins?

King George V of Great Britain, Kaiser Wilhelm II of Germany and Czar Nicholas II of Russia.

George V and Wilhelm II were both grandsons of Queen Victoria of Britain. Wilhelm was in fact her favorite grandchild, and she died in his arms. The wife of Nicholas of Russia was also a grandchild of Victoria.

Nicholas was cousin by way of his mother, who was a younger sister of the mother of George V.