Explanation: Amongst the royals of Ancient Egypt, there was a curious practice of sibling incest, wherein brothers and sisters were married to keep the bloodline 'pure'.
This strange practice eventually spread to some of the broader population as well. It was noted during the Roman Empire that Egyptians still sometimes married their siblings - something unacceptable to Roman norms, but was left uninterfered with because... well, provincials aren't subject to Roman norms.