According to Wikipedia incest was practiced by all social classes in ancient Egypt from the Pharaoh downwards. The article then lists varying degrees of taboo or permissiveness attached to incest by other Classical era cultures. How did these other cultures view the Egyptians, who seem to have taken the practice the farthest?
As an aside, is there any evidence that the ancient Egyptians suffered from more genetic disorders due to widespread incest?
I'm afraid I can't answer this question, but I wanted to give you a quick correction. (Mods, I hope this is okay.) You'll note that Wikipedia says, specifically, "It is now generally accepted that sibling marriages were widespread among all classes in Egypt during the Graeco-Roman period." This was only one period, a fairly late one, in a very long history.
Incest was not "practiced by all social classes in ancient Egypt from the Pharaoh downwards." It's generally accepted that throughout the majority of Egyptian history, incest was practiced by the royal family, likely to keep the bloodline "pure" and make sure the royal group was exclusive, while incest was not practiced by common, everyday folk. (However, on an interesting note, in ancient Egyptian romantic poetry, "brother" and "sister" are used as terms of endearment between lovers.)
Incest became a more widespread practice in the Roman period because of complex inheritance laws that made brother-sister marriages the easiest way to keep property within the family.