Of course one first cousin marriage every 5 generations is about equivalent to a 5th cousin marriage every generation. The Pharaohs had dynasties of several hundred years based on sibling marriage. The recessive genes would be filtered out rather quickly by infanticide. The Hawaiian royalty also had sibling marriage. We are all pretty closely related, with our DNA being more than 99% common just by being human. It is likely that some of our visual diversity is due to a different corps of parasites living in/on us, and not so much our DNA variation.
Hmmm...there are counterexamples.