Personally, with Abraham and Sarah, I do believe that he was definately into the habit about lying on her identity...and I think there are other options present other than her being his neice.
Truthfully, as Abraham was noted to have lied on many occassions, part of me has often wondered if perhaps what he said about Sarah being his sister was also a lie. Within Genesis 11, one can see the names of the children that Abraham's father had...and when it comes to mentioning their wives, what the text says is that Abrahm, Nahor and Haran were Terah's children. Haran died while the other two sons married...and only Nahor's wife, Milcah, was noted to be the daughter of Nahor's brother (Haran) in Genesis 11:29. But never is it mentioned about where Sarah came from..and it never says she was Terah's daughter from his wife. I've been of the mindset lately that Abraham was lying about Sarah coming from Terah just as he had lied earlier (And was notorious for doing).
Sarah could've easily been a woman from the other nations and married into the family....and Abraham used the lie of her being his "half-sister" or "sister" out of convience just as he often did error on other things.
And with anyone having concern with bringing Abraham's character into question, IMHO, one may wish to consider how he already seemed to be in the habit of treating others in stressful situations. Abimelech, whom he lied to in the first place, was a righteous man....and with a righteous man there was no need assuming that he'd treat Sari shamefully---or be unable to handle the truth in her being Abraham's husband. Abraham's comment in Genesis 20:11---where he stated "I did it because there is no fear of God in this place..."---betrays both his lack of faith in God and his misjudgement of the people of Geerar. The whole episode reveals that the King and His servants were God-fearing, as seen in Genesis 20:8 and Genesis 20:16-18 when Abimelech was exceedingly gracious/generous. His generosity, on top of his innocence, contrasts sharply with Abraham's self-serving deception regarding the truth about Sarah...for the king's actions were a very public affirmation that he had not acted inappropiately toward Sarah...and thus, he was not the father of any child she had. Abraham was shown in Genesis 20:12-13 to really have been a greater sinner than Abimelech. For when he says "at every place to which we have come I've said she was my sister", Abraham showed that he REGULARLY resorted to the wife-sister ruse for his own self protection. Genesis 12 and 20 seem to reveal that it did not always work...and only God's intervention protected Abraham's relationship with Sarah
Abraham had used the same trick before to protect himself/his wife from others whom he automatically chose not to trust (Genesis 20:2)...despite how the Lord had already told Abraham that He would look out for them. Although Abraham is one of our heros of the faith, it seems he did not learn his lesson well enough the first time. In fact, by giving into temptation to lie in order to protect his wife/himself, he risked turning a sinful act into a a sinful pattern of lying whenever he suspected his life was in danger. And it literally placed the lives of others in danger that should have never been in such had he simply been honest. Because Abraham mistakenly/rashly assumed that Abumelech was a wicked man, he made a quick decision to tell a half-truth Abraham thought it would be more effective to deciecve Abimelech than to trust God to work in the King's life......and the all the wombs of innocents in the house of Abimelech were closed up. It was, of course, a protection given by God so that Abimelech would catch the picture and see he was in danger......for it was meant to change the situation rather than harm Abimelech.
In some ways, it seems Abraham struggled with trusting others who were worthy of it as many do today when they'll remember instances where others endangered them---but then sabatouge all other relationships that are good...........even those who are on their side. The fact that God came through doesn't mean it was due to Abraham's actions---as many times, God will work DESPITE a problem rather than BECAUSE of it---just as He did with the mistake of Hagar/Ishmael in Genesis 16 and Genesis 21:8-20..which happened directly before the episode with Abimelech. God in His Mercy PREVENTED Abimelech for committing great sin due to the actions of another who sinned as well in misjudging without cause. Sometimes, one can tempt the Lord and place Him in positions where he MUST intervene even when He was not desiring to do so in a prescribed way. Its like people who don't study for a test for driving...and then when driving, they have others in the car whose lives are now at risk. Their praying and trusting God to help them doesn't mean God approved of their actions when he keeps them from a wreck....for they were still called to study/train. And due to lacking it, others almost got killed
Going back to the identity of Sarah, some have suggested that Sarah's identity was that of Iscah...who is mentioned only once in Scripture.
Gen 11:29
Then Abram and Nahor took wives: the name of
Abram's wife was Sarai, and the name of Nahor's
wife, Milcah, the daughter of Haran the father
of Milcah and the father of Iscah.
In this verse every other name mentioned is of biblical significance, so the name 'Iscah' may also refer to someone of some biblical relevance. Observing that her sister, Milcah, had married Nahor (Abraham's brother) many have thought Iscah may be merely another name for Sarai (ie Sarah), Abraham's wife. That is to say, that each brother had married a daughter of their other brother Haran.
However, the meaning of the name 'Iscah', according to Strong's Concordance, is 'to watch' or 'observant'. Those who have thought Iscah was Sarah have considered the meaning of the name may refer to the well documented beauty of Sarah.
There are some good reasons as to why Iscah is probably not Sarah. A critical text which has to be considered is Abraham's explanation of why his wife was also his sister.
Gen 20:12
But indeed she is truly my sister. She is the daughter of my father, but not the daughter of my mother; and she became my wife.
This verse suggests that Abraham's father, Terah, was married to at least two women. To a lady who was the mother of Abraham and to another lady who was the mother of Sarah. Since the word 'father' in Hebrew can also refer to a grandfather, great grandfather, etc, and 'mother' can also refer to a grandmother, great grandmother etc., and 'daughter' can also refer to a grand daughter etc. , other possibilities exist.
Although it's possible Sarah could be the niece of Abraham, such as Iscah was, it's also possible that Sarah could also be a younger daughter of one of Abraham's male ancestors provided there was a different mother. In interpreting Gen 20:12 we should also seriously consider the marriage laws in Lev 18:6-16 (repeated in chapter Genesis 20). While these laws were later documented as part of the Old Covenant regulations, it is also likely they were requirements expected to be adhered to in Abraham's time.
Thus, the regulation against one marrying either the daughter of one's father would suggest Sarah was not Terah's biological daughter.
Some suggest Sarah could not have been a half sister of Abraham. ..for in verse Gen 20:12 we also find mentioned that Sarah was truly Abraham's sister. Without a common biological parent the notion of 'sister' would suggest they both grew up in the very same family unit. Hence, while not biologically related, they would still regard themselves as truly brother and sister....similar to others today who grow up in families that are involving others together not biologically related and yet seeing themselves as "siblings".
I've had many relationships like that and it's a trip - and the same thing goes for noting to others who are my aunts/uncles even though they're not biological since they fulfill a role (more shared here in #24 ).