And yet indeed she is my {m} sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.

(m) By sister, he means his full cousin, and by daughter Abraham's niece, Ge 11:29 for so the Hebrews use these words.

Jamieson-Fausset-Brown Bible Commentary

12. yet indeed she is my sister-(See on [8]Ge 11:31). What a poor defense Abraham made. The statement absolved him from the charge of direct and absolute falsehood, but he had told a moral untruth because there was an intention to deceive (compare Ge 12:11-13). "Honesty is always the best policy." Abraham's life would have been as well protected without the fraud as with it: and what shame to himself, what distrust to God, what dishonor to religion might have been prevented! "Let us speak truth every man to his neighbor" [Zec 8:16; Eph 4:25].

Matthew Henry's Concise Commentary

20:9-13 See here much to blame, even in the father of the faithful. Mark his distrust of God, his undue care about life, his intent to deceive. He also threw temptation in the way of others, caused affliction to them, exposed himself and Sarah to just rebukes, and yet attempted an excuse. These things are written for our warning, not for us to imitate. Even Abraham hath not whereof to glory. He cannot be justified by his works, but must be indebted for justification, to that righteousness which is upon all and unto all them that believe. We must not condemn all as hypocrites who fall into sin, if they do not continue in it. But let the unhumbled and impenitent take heed that they do not sin on, thinking that grace may abound. Abimelech, being warned of God, takes the warning; and being truly afraid of sin and its consequences, he rose early to pursue the directions given him.