There's a running joke in Maringouin, Louisiana, a town of 1,100, that everyone is related. It's funny because, as people in Maringouin will tell you, it's true. Everyone calls each other 'cuz' or 'cousin,' and they mean it. People run into each other on the street, recognize a last name, start talking about people they know in common, then discover they're related.
For a long time, no one knew exactly why. It wasn't like there was a founding family that had moved there.