Genesis 20:2

She is my sister - See the parallel account, Genesis 12 (note), and the notes there. Sarah was now about ninety years of age, and probably pregnant with Isaac. Her beauty, therefore, must have been considerably impaired since the time she was taken in a similar manner by Pharaoh, king of Egypt; but she was probably now chosen by Abimelech more on the account of forming an alliance with Abraham, who was very rich, than on account of any personal accomplishments. A petty king, such as Abimelech, would naturally be glad to form an alliance with such a powerful chief as Abraham was: we cannot but recollect his late defeat of the four confederate Canaanitish kings. See note on Genesis 14:14, etc. This circumstance was sufficient to establish his credit, and cause his friendship to be courted; and what more effectual means could Abimelech use in reference to this than the taking of Sarah, who he understood was Abraham's sister, to be his concubine or second wife, which in those times had no kind of disgrace attached to it?