John Calvin on Genesis 20:12

12. And yet indeed she is my sister . Some suppose Sarah to have been Abraham’s own sister, yet not by the same mothers but born from a second wife. As, however, the name sister has a wider signification among the Hebrews, I willingly adopt a different conjecture; namely, that she was his sister in the second degree; thus it will be true that they had a common father, that is, a grandfather, from whom they had descended by brothers. Moreover, Abraham extenuates his offense, and draws a distinction between his silence and a direct falsehood; and certainly he professed with truth, that he was the brother of Sarah. Indeed it appears that he feigned nothing in words which differed from the facts themselves; yet when all things have been sifted, his defense proves to be either frivolous, or, at least, too feeble. For since he had purposely used the name of sister as a pretext, lest men should have some suspicion of his marriage; he sophistically afforded them an occasion of falling into error. Wherefore, although he did not lie in words, yet with respect to the matter of fact, his dissimulation was a lie, by implication. He had, however, no other intention than to declare that he had not dealt fraudulently with Abimelech; but that, in an affair of great anxiety, he had caught at an indirect method of escape from death, by the pretext of his previous relationship to his wife.