Actually, if you look at the history of the patriarchs and at the actual laws in Leviticus 18 and 20, the sin was not incest as we know it. The Bible is not concerned with the problems inherent in high degrees of consanguinity. Abraham and Sarah are claimed to be brother and sister (or at least half-brother and half-sister) -- although some apologists are quick to point out that the phrase used can sometimes be used of close cousins raised as siblings. Isaac and Rebbecca were first cousins, as were Jacob and both Leah and Rachel. Both Leverate marriage and laws limiting the choices of the priests and Levites were intended to keep the priestly and later the royal blood as pure and strong as possible.
Leviticus 18 and 20, were not about shared blood. Some of the forbidden liasons were with the uncle's (unrelated) wife, and a sister whether natural or adopted. There were two concerns. One, adultery was especially heinous when it was within the family, bringing a double measure of shame on the cuckolded husband.
And two, since an unmarried woman gets her status from her father, but a married woman gets it from her husband, a woman's status within the family can suddenly and dramatically change after marriage. There can be contention within the family when the status quo between two members suddenly changes. This is the specific reason given in Leviticus as to why marriage to two sisters or other closely related women was discouraged. It is also the subject of several proverbs that speak of marital peace, or more to the point, the lack of peace in a marriage.
