When you think about it, World War I looks like a family feud between the Kaiser, the Tsar, and I believe it was the Duke of York at the time. They were all grandsons of Queen Victoria.
And millions of people died as a result of that.
The grand idea was that somehow intermarriage of the various royal families would simmer things down. But the royalty didn't necessarily have any real power. Ultimately, nations don't have friends and relatives, they have interests.