Genesis 20:11–12
And Abraham said, “Because I thought, surely the fear of God is not in this place; and they will kill me on account of my wife. But indeed she is truly my sister. She is the daughter of my father, but not the daughter of my mother; and she became my wife.

Deuteronomy 27:22
‘Cursed is the one who lies with his sister, the daughter of his father or the daughter of his mother.’ And all the people shall say, ‘Amen!’ [Also see Leviticus 18:6–9.]

These two passages, to some, present quite a contradiction. Genesis tells us that Abraham, a man blessed by God, married his half-sister, while Moses in Deuteronomy tells the Israelites that anyone who lies with his sister shall be cursed. So, is this a contradiction in the Bible? Not at all!

To understand why, we need to start at the beginning with Adam and Eve. We don’t know how many children Adam and Eve had, but the Bible makes it clear that they had other “sons and daughters” in addition to Abel, Cain, and Seth (Genesis 5:4). So, for the human population to increase, and for them to be obedient to God’s command to be fruitful and multiply, brother and sister had to get married. There were no others around! But doesn’t this violate God’s commands that prohibit close relations marrying? Certainly not.

We need to keep in mind that the Law of Moses wasn’t given until around 1440 BC, more than 2,500 years after creation and more than 400 years after Abraham’s time. So, why did God institute laws against marrying close relatives? The following section from the chapter “Cain’s Wife—Who Was She?” in The New Answers Book explains why:

The more closely related two people are, the more likely it is that they will have similar mistakes in their genes, inherited from the same parents. Therefore, brother and sister are likely to have similar mistakes in their genetic material. If there were to be a union between these two that produces offspring, children would inherit one set of genes from each of their parents. Because the genes probably have similar mistakes, the mistakes pair together and result in deformities in the children.

. . . However, this fact of present-day life did not apply to Adam and Eve. When the first two people were created, they were perfect. Everything God made was “very good” (Genesis 1:31). That means their genes were perfect—no mistakes. But when sin entered the world because of Adam (Genesis 3:6), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (Romans 8:22). Over a long period of time, this degeneration would have resulted in all sorts of mistakes occurring in the genetic material of living things.

. . . By the time of Moses (about 2,500 years later), degenerative mistakes would have accumulated to such an extent in the human race that it would have been necessary for God to bring in the laws forbidding brother-sister (and close relative) marriage (Leviticus 18–20).

So, once again, we see that there are no contradictions in the Scriptures when we take the time to study them in their proper historical and theological context. http://www.answersingenesis.org/articles/2009/05/04/contradictions-too-close-for-comfort