That brother/sister procreation thing really worked out well for the royal family of ancient Egypt, didn't it?
They were conquered by Macedonian Greeks and then wiped out by the Roman Empire.
Inbreeding does not work well.
One of the big problems in the Arab countries is the incredible amount of inbreeding. Not brother and sister, but many, many cousins intermarry. Men and women are not allowed to socialize in an open environment, so they tend also to marry within the family.
That is reflected in a strong clannish mentality, and lack of any tolerance for anyone who is not in your particular grup.
I tried to tell that to a member of the KKK a while back. But of course I used the words "racial purity" instead.... Same results though, just look at the skin heads/KKK/neo-nazi's now and you'll realize where racial purity leads to.
-Traveler