Abraham married his sister:
11 Abraham said, “I did it because I thought, ‘There is no fear of God at all in this place, and they will kill me because of my wife.’ 12 Besides, she is indeed my sister, the daughter of my father though not the daughter of my mother, and she became my wife. [Genesis 20:11-12 ESV]
And God blessed this marriage:
15 And God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name. 16 I will bless her, and moreover, I will give you a son by her. I will bless her, and she shall become nations; kings of peoples shall come from her.” [Genesis 17:15-16 ESV]
However, incestuous relationships are explicitly prohibited in Scripture:
22 “‘Cursed be anyone who lies with his sister, whether the daughter of his father or the daughter of his mother.’ And all the people shall say, ‘Amen.’ [Deuteronomy 27:22 ESV]
17 “If a man takes his sister, a daughter of his father or a daughter of his mother, and sees her nakedness, and she sees his nakedness, it is a disgrace, and they shall be cut off in the sight of the children of their people. He has uncovered his sister's nakedness, and he shall bear his iniquity. [Leviticus 20:17 ESV]
How can we reconcile all this?