Despite the deceit, the Bible still states that Sarah is a close blood relation of Abraham, specifically his half sister, and hence many modern cultures (and even the Bible itself, see Leviticus 18:9) would view her also being his wife as a form of incest. However, in some significant ancient cultures, such as the Hurrian and Egyptian cultures, biological sisters were often raised to the position of a wife in order to give the sister a greater standing in society, and this tradition is one with which Abraham would be likely to have come into contact during his migration.
On the other hand, there has been ancient tablets recently recovered from the ancient city of Mari that suggest otherwise. These ancient Semite legal records show that when a woman is married to a man, she is then formally adopted by his father as a full daughter as well[1]. Like Abraham, many ancient Semites were Nomads and it was customary for the daughter-in-law to be officially adopted as a full daughter in case her husband is to die while she is traveling with his family.
According to Genesis 12.5, Sarah left her family to set out for the land of Canaan, which puts her in this same position as suggested in the ancient tablets of Mari (a Semite city which Abraham is presumed to have visited). This suggests that Sarah was not Abrahams half-sister, but adopted sister by law. Thus, Abraham did not lie, nor did he commit incest. However, there is little question that the deception was intentional.
Still, many believe that the verse concerning the 'cover of the eye' is probably instructions that Sarah should wear a veil in order to show her marital status, as was the custom of many ancient pagan cultures. By doing this she would clear up any confusion as to her availability (yet still be able to claim that Abraham his her brother).