incest between cousins was always quite common up until relatively recently, you would have thought that that would have weeded out a lot of the bad genes
With cousins It's still very common in a lot of near eastern cultures...as a student of Arabic told me, one practice phrase in the textbook was something like, "in fact, my brother in law also happens to be my cousin," which got the class buzzing but his professor said was pretty normal.
Later in the same book (Genesis), Abraham (and later his son with his own wife) have a perennial cowardice problem demonstrated by telling kings whose cities he's passing through that Sarah his wife is his sister, so the kings won't kill him and take her. The justification Abraham gives at one point is that it's a half-truth, because she really is his half sister. (20:12-13) http://www.biblegateway.com/passage/?search=Genesis+20&version=NIV As far as the author is concerned, at that early point in history, the only moral problem seems to be the half-truth and the risk of adultery it creates, not the incestuous relationship itself, which is one of the most genealogically important and sacred marriages in the Abrahamic faiths. Still, brother-half-sister relationships (not to mention, obviously, the "Abel and Eve" variety) were banned later(Lev. 18). Some degree of endogamy seems to have been preferable in maintaining family identity in the new land (see Abraham making sure his son gets a wife from his own relatives back in the old country in Gen. 24).
It's interesting that in explaining the half-truth Abraham also claims calling herself his "sister" is supposed to be a sign of love, which may have stayed sexy in that culture, given it's a sort of pet name also used a lot for the bride in the Song of Songs.
It seems like a bit of a stretch how, as I recall, some evolutionary psychologists, in trying to counter sociology's and anthropology's cultural relativist assumptions, have listed "incest taboo" as one of the universals they see across cultures, maybe not totally inaccurate but much less clear-cut than we think.