According to an extra-Biblical source Abraham grew up in the house of Shem learning God’s Law. Abraham knew it was wrong to marry his sister. The answer is that Abraham actually married his niece.
Genesis 11:29 with a change of one letter. “And Abram and Nahor took them wives: the name of Abram’s wife was Sarai; and the name of Nahor’s wife, Milcah, the daughters of Haran, the father of Milcah, and the father of Iscah.”
The change of the word daughter to daughters is a valid grammatical adjustment in the Hebrew. Also there was no Hebrew word for nephew or niece so brother and sister are used. For more about the Scripture recommended reading, The Book of Jasher, see http://www.gospelminutes.org/jasher.php