We are told in previous studies that Sarah/Sarai, was Abraham’s sister, both having the same father, but Sarah had a different mother (Genesis 20:12). This would make Tehah a bigamist, and Sarah/Sarai the daughter of a different wife of Terah than Abraham’s mother. Abraham was born of one and Sarah the other. In other words, Genesis 20:12 may indicate that by the time we come to Abraham’s day folks married within the same family, but only in the case of the marital pair having different mothers. If this is so, then we could assume that Abram/Abraham and Nehor were full brothers, having the same father and mother, and Haran and Sarai/Sarah were brother and sister and had the same father and mother. Such an understanding permits Abraham and Jacob to later seek wives from the descendants of Nahor, Abraham’s brother.
Of course, this is conjecture, and it may be that marrying one’s sister during Abraham’s day didn’t matter that the marital pair had the same mother or not. Indeed, in the beginning there were no other choices except for Adam’s sons to take Adam’s daughters as their wives (Genesis 5:4; cp. 4:16-17). Nevertheless, if such things didn’t matter during Abraham’s day, why would Abimelech and Pharaoh naturally assume that Sarai/Sarah was available for marriage (Genesis 12:18-19; 20:2)? Indeed, why would Abraham naturally assume that calling Sarai/Sarah his sister would protect him from being slain by a stronger foe in the event such a one would want to take Sarai/Sarah for his wife (Genesis 12:10-13; 20:11-12)? Moreover, in the matter of Lot and his daughters, why would the daughters of Lot believe they had to get their father drunk before he would give them children by himself (Genesis 19:30-38), if full-blooded incest wasn’t a matter of concern in Abraham’s day?
So, before Abraham left the land of Ur in Mesopotamia (Genesis 11:31), we are told that, for all intents and purposes, Abraham’s eldest brother, Haran, was offered up as a human sacrifice by Terah their father (Genesis 11:28).[1] After this event, both Abram/Abraham and Nahor took wives for themselves. Abram/Abraham took Sarai/Sarah, and Nahor took Milcah, who was the daughter of Haran. Thus, the context of taking one’s wife from one’s own family might point to Sarai/Sarah as the full-blooded sister of Haran. In this way, and for a reason unknown, in as much as I can tell, Terah was uniting his two sub-families into one.
Returning, now, to the time just after Abraham returned to Beersheba from Mount Moriah, he received word from his brother, Nahor, that Milcah had borne children unto him, eight sons and one grandson by Kemuel, and one granddaughter, Rebekah, by Bethuel (Genesis 22:20-23). He also had a concubine who bore four other sons to him (Genesis 22:24).
In the context of this study, both Abraham and, later, Jacob felt free to draw wives from Nehor’s clan. Rebekah, of course, was taken by Abraham for his son, Isaac. However, Jacob requested Rachel for his wife from her father, Laban, Rebekah’s brother and son of Bethuel, the son of Abraham’s brother, Nahor (Genesis 22:23; 24:29), and Jacob not only received Rachel for his wife, but also her sister Leah (Genesis 29:18-28). So, at least during the time of Abraham, it wasn’t considered incest to marry one’s near relative, provided, I presume, different mothers were the matrons of the family trees, from which the two wanting to marry descended.
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[1] See my previous study: The Sacrifice of Isaac.
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