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This article means nothing to me unless I have some means to compare current and past trends. In particular raising the case of Kennedys isn't too helpful, because that's been going on for half a century already, which is a big fraction of the existence of this nation.
Making matters worse, a schoolchild can easily come up with examples from the past. Quincy Adams was the son of John Adams. Benjamin Harrison was the grandson of William Henry Harrison. TR and Eleanor Roosevelt were related, and FDR more distantly. That's six presidents and a president's wife right there.
There is nothing clear about this supposed trend.