“Besides, she actually is my sister, the daughter of my father, but not the daughter of my mother, and she became my wife; and it came about, when God caused me to wander from my father’s house, that I said to her, ‘This is the kindness which you will show to me: everywhere we go, say of me, “He is my brother.”’ Gen. 20:12-13
Abraham here defends his statement about Sarah to Abimelech, king of Gerar. Because Abraham is generally held in high esteem by both Jews and Christians, most hold that he was not lying, but rather simply de-emphasizing his marital relationship over the familial when he felt his life was in danger. In both cases the circumstances were precipitated by a combination of Sarah’s beauty along with a perceived lack of morals among the local people.
Genesis 20:12 is the only verse in scripture that supports Sarah being Abraham’s half-sister. We do not otherwise see a family lineage of Sarah. There is speculation that Sarah could actually be Iscah, Haran’s daughter (Abraham’s brother, see Gen. 11:29), which would technically make Sarah Abraham’s niece, a little closer to what Abraham was claiming. However it is only speculation based on Iscah’s name, which could mean ‘to gaze’ (on account of Sarah’s beauty). Rashi proposed that Sarah was Iscah because Iscah implies aristocracy, and Sarah is generally thought to mean ‘princess‘.
Marrying a half-sister may have been culturally acceptable in the Mesopotamian region where Abraham came from, but was likely not acceptable in Egypt or Gerar, thus the ruse was concocted and agreed to by Sarah, as we see in verse 13. Later, we see that the Levitical law has restrictions on closeness between certain blood relatives, including sisters, whether born from the father or mother (Lev. 18:9).
It is quite obvious from the narrative here that ultimately God considers the marital relationship to supersede any blood relationship, as He even threatens death to Abimelech if he does not return Sarah to Abraham. Genesis 2:24 reads “For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh.” This illustrates marriage is to be considered the strongest of all relational bonds between humans.