The heads of three of the main warring countries at the outbreak of WW1 were cousins.

The picture shows Tsar Nicholas II of Russia with his physically similar cousin, King George V of the United Kingdom (right), in German military uniforms in Berlin about a year before World War 1. There were other important connections: Nicholas' wife Alexandra and Kaiser Wilhelm II of Germany were also first cousins of King George.

One cannot help but think that if there had been better communications within the extended family the 20th century might have been spared a lot of grief.

To try to sort out the complex relationships read this: https://en.wikipedia.org/wiki/Nicholas_II_of_Russia