This topic was mentioned in another thread and 'coincidently' I had been looking into this subject very recently, although I make no claim in having a definitive answer to it I do have some observations that I thought might make for an interesting thread and perhaps inspire some of us to dig deeper into our bibles.
First of all Abraham predated Moses and the Levitical laws by over 600 years, perhaps I missed it but I found no scripture that predates Lev 18:9 or Lev 20:17.
Lev 18:9 The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover.
Lev 20:17 And if a man shall take his sister, his father's daughter, or his mother's daughter and see her nakedness and she see his nakedness; it is a wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity.
With that being said I am not making any claim that Sarah and Abraham were indeed brother and sister or half brother and sister, as a matter of fact I lean toward the idea that Abraham was telling a 'white lie' or being purposely ambiguous so as to keep himself alive (but risk losing his wife). There are two places where Abraham (as Abram first) makes this statement.
Gen 12:11 And it came to pass, when he was come near to enter into Egypt, that he said unto Sarai his wife, Behold now, I know that thou art a fair woman to look upon:
Gen 12:12 Therefore it shall come to pass, when the Egyptians shall see thee, that they shall say, This is his wife: and they will kill me, but they will save thee alive.
Gen 12:13 Say, I pray thee, thou art my sister: that it may be well with me for thy sake; and my soul shall live because of thee.
Gen 12:14 And it came to pass, that, when Abram was come into Egypt, the Egyptians beheld the woman that she was very fair.
Gen 12:15 The princes also of Pharaoh saw her, and commended her before Pharaoh: and the woman was taken into Pharaoh's house.
Gen 12:16 And he entreated Abram well for her sake: and he had sheep, and oxen, and he asses and menservants and maidservants and she asses and camels.
Gen 12:17 And the LORD plagued Pharaoh and his house with great plagues because of Sarai Abram's wife.
Gen 12:18 And Pharaoh called Abram and said, What is this that thou hast done unto me? why didst thou not tell me that she was thy wife?
Gen 12:19 Why saidst thou, She is my sister? so I might have taken her to me to wife: now therefore behold thy wife, take her, and go thy way.
Gen 12:20 And Pharaoh commanded his men concerning him: and they sent him away and his wife, and all that he had.
We see that it all turns out well this first time, the Lord covered Abram's deception with no harm done to either him or his wife (at least at that time), did Abram show a lack of faith in God by lying instead of telling the truth or is/was Sarai indeed his half sister by blood or by a careful manipulation of words?
Well it worked once, why not again?
Genesis 20
1And Abraham journeyed from thence toward the south country, and dwelled between Kadesh and Shur, and sojourned in Gerar.
2And Abraham said of Sarah his wife, She is my sister: and Abimelech king of Gerar sent, and took Sarah.
3But God came to Abimelech in a dream by night, and said to him, Behold, thou art but a dead man, for the woman which thou hast taken; for she is a man's wife.
4But Abimelech had not come near her: and he said, LORD, wilt thou slay also a righteous nation?
5Said he not unto me, She is my sister? and she, even she herself said, He is my brother: in the integrity of my heart and innocency of my hands have I done this.
6And God said unto him in a dream, Yea, I know that thou didst this in the integrity of thy heart; for I also withheld thee from sinning against me: therefore suffered I thee not to touch her.
7Now therefore restore the man his wife; for he is a prophet, and he shall pray for thee, and thou shalt live: and if thou restore her not, know thou that thou shalt surely die, thou, and all that are thine.
8Therefore Abimelech rose early in the morning, and called all his servants, and told all these things in their ears: and the men were sore afraid.
9Then Abimelech called Abraham, and said unto him, What hast thou done unto us? and what have I offended thee, that thou hast brought on me and on my kingdom a great sin? thou hast done deeds unto me that ought not to be done.
10And Abimelech said unto Abraham, What sawest thou, that thou hast done this thing?
11And Abraham said, Because I thought, Surely the fear of God is not in this place; and they will slay me for my wife's sake.
12And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.
13And it came to pass, when God caused me to wander from my father's house, that I said unto her, This is thy kindness which thou shalt shew unto me; at every place whither we shall come, say of me, He is my brother.
14And Abimelech took sheep, and oxen, and menservants, and womenservants, and gave them unto Abraham, and restored him Sarah his wife.
15And Abimelech said, Behold, my land is before thee: dwell where it pleaseth thee.
16And unto Sarah he said, Behold, I have given thy brother a thousand pieces of silver: behold, he is to thee a covering of the eyes, unto all that are with thee, and with all other: thus she was reproved.
17So Abraham prayed unto God: and God healed Abimelech, and his wife, and his maidservants; and they bare children.
18For the LORD had fast closed up all the wombs of the house of Abimelech, because of Sarah Abraham's wife.
Let's look closer at Gen 20:12;
Gen 20:12 And yet indeed she is my sister;269 she is the daughter of my father,1 but not the daughter of my mother; and she became my wife.
Are we witnessing the self justification of Abraham, continuing this deceit to save face with Abimelech? Here are Strong's definitions of the Hebrew words for sister and father;
H269
אחות
'âchôth
aw-khoth'
Irregular feminine of H251; a sister (used very widely (like H250), literally and figuratively): - (an-) other, sister, together.
H1
אב
'âb
awb
A primitive word; father in a literal and immediate, or figurative and remote application: - chief, (fore-) father ([-less]), X patrimony, principal. Compare names in “Abi-”
Was Abraham (cleverly) referring to himself Sarah as both being children of God thus "the same father"?
My perception or present understanding is no matter what our sin(s) God has covered us in the Blood of His Son, we all are guilty yet His provision gives us life rather than a death sentence, even when we lack faith He provides, even when we take the path of the least resistance instead of taking the higher more difficult road He forgives, whether we are purged in this life or in judgment we are all destined to be heirs of His glory.
Will Abraham and Sarah be judged for these acts in the second resurrection?
Heb 11:39 And these all, having obtained a good report through faith, received not the promise:
Heb 11:40 God having provided some better thing for us, that they without us should not be made perfect. (Read this entire chapter for more depth and clarity).
I think we all too often ascribe greater attributes to the Prophets and Apostles than what was the reality, they were all frail and vain beasts without the Spirit directing and inspiring them, we are all the same in that regard. Without God's Spirit we are only self righteous and selfish sinning machines.
Peace,
Joe