I thought it strange that Abraham claimed Sarah was his sister twice and then Isaac did the same thing. Apparently, they both feared for their lives, (rather than laying down their lives for their wives). Abraham had experienced great military victories in the past, too, so it is hard to conceive he was fearful of anything or anyone. Was there a middle east custom whereby a husband would be killed to obtain his wife?
According to Genesis 20:12, Sarah was Abraham’s half sister as well as his wife. She was the daughter of his father but not of his mother. Isaac’s connection to Rebekah was a little more complicated. Her grandmother was Milcah, who was both Abraham’s sister-in-law and his niece (Genesis 11:27-29). That means Isaac was both Rebekah’s uncle (by marriage) and her great uncle (by birth).
There was no divorce in those days. The only way for a king to take a man’s wife was to have the man killed, making his wife a widow and therefore eligible. (2 Samuel 11 tells how King David did this so he could marry Bathsheba.)
Both Abraham (Genesis 12:12-13) and Isaac (Genesis 26:7) were afraid of being killed for this reason. The probability of this happening is especially remarkable in the case of Sarah who was only 10 years younger than Abraham (Genesis 17:17).
Genesis 12:4 indicates Abraham was at least 75 years old when he finally arrived in the promised land. That would have made Sarah 65. Sometime after that they went to Egypt where Sarah’s beauty so impressed Pharaoh that he took her into his palace (meaning his harem). It would have been unheard of for a ruler who could have any woman he desired to choose one in her late 60’s.
I believe God retarded the aging process for both Abraham and Sarah to allow them to give birth to Isaac at such an advanced age. He was 100 when Isaac was born and Sarah was 90. Sarah died at age 127, but Abraham married again and had six more sons (Genesis 25:1-2) finally dying himself at age 175 (Genesis 25:7).