This is a matter of state law. The states, of course, vary. Some cultures allow that level of relationship and others don't. In Jewish law an uncle can marry his niece but an aunt cannot marry her nephew. There are rationalizations but no actual science behind any of these laws, especially as marriage is not legally dependent upon having children. Louisiana law outlaws marriages between first cousins (although it used to be legal), as do about half the other states but few other countries. Aunts and uncles etc can't marry nieces and nephews in Louisiana either, unless it was recently changed.  Much of this sort of law in the USA derives from interpretations of the Bible.
Source(s):
http://volokh.com/posts/1224522825.shtml
http://books.google.com/books?id=0r4SAAAAYAAJ&pg=PT201&lpg=PT201&am...

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