"The First World War has sometimes been labelled, with reason, "a family affair". This is derived from the reality that many of the European monarchies - many of which fell during the war (including those of Russia, Germany and Austria-Hungary) - were inter-related.
The British monarch George V's predecessor, Edward VII, was the German Kaiser's uncle and, via his wife's sister, uncle of the Russian Tsar as well. His niece, Alexandra, was the Tsar's wife. Edward's daughter, Maud, was the Norwegian Queen, and his niece, Ena, Queen of Spain; Marie, a further niece, was to become Queen of Romania.
Despite these familial relations - nine Kings attended Edward's funeral - European politics was all about power and influence, of protection and encirclement. Thus the tangled web of alliances which sprung up in the wake of the rise of the newly united German Empire in 1871."
From...
http://www.firstworldwar.com/origins/causes.htm