Since we are all related, this is really not so surprising

Pedigree Collapse or Consanguinity

Some people fall in love and marry their childhood sweethearts. Others marry their neighbors. And still others marry their cousins. Our family is no different than yours. You are invited to take a look at how this works and some specific examples.

If you wish some additional light reading on this topic there are a couple of great examples here: and here: and this article discusses the phenomenum as it applies in the United States