It is a well known fact that in ancient Egypt, it was not unknown for a Pharaoh to marry his own sister.
As such, I have 2 questions (about pre-Alexander/Ptolemaic times):
How common/uncommon was such a marriage for Pharaohs?
Was the same lax attitude extended to non-Pharaohs, especially "normal" people? Or were they prohibited from sibling marriage like a vast majority of other societies, not being "gods"?
It is generally accepted that sibling marriages were widespread among all classes in Egypt during the Graeco-Roman period. Numerous papyri and the Roman census declarations attest to many husbands and wives being brother and sister.
Here is a large article on the subject of incest in ancient Egypt. It seems, that in the Old Kingdom it was not in practice, and during Middle Kingdom it was very rare. But we have no knowledge about limitations during this period.
It was an accepted practice for the Pharaohs of Ancient Egypt. The pharaohs had entire area's of the palace set aside for the children of their concubines etc. that were not considered of royal blood (those of royal blood studied along side them for a time). There they studied and were tutored for years and eventually became officials or priests. Incest, harems, and premarital sex all happened in Egypt and it appears that everyone except Pharaoh were expected to be monogamous.
You can read some about it here: http://fathom.lib.uchicago.edu/2/21701778/
There are also some really good books on the subject if you look around on the web.