Catena Fidei

Genesis 20:12 (WEB)
Besides, she is indeed my sister, the daughter of my father, but not the daughter of my mother; and she became my wife.

Fathers and councils · 4

Augustine of Hippo, Against Lying Chapter 23
“For he did not say, She is not my wife, but he said, “She is my sister;” because she was in truth so near akin, that she might without a lie be called a sister.” — editor's note: Gen. xx. 2, 12
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Augustine of Hippo, The City of God XVI.30Book XVI
“…his sister, he explained what he had been afraid of, and added this further, “And yet indeed she is my sister by the father’s side, but not by the mother’s; for she was Abraham’s sister by his own father, and so near of kin.” — editor's note: Gen. xx. 12.
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Clement of Alexandria, The Stromata, or Miscellanies II.23Book II
“Hence Abraham, regarding his wife as a sister, says, “She is my sister by my father, but not by my mother; and she became my wife,” teaching us that children of the same mothers ought not to enter into matrimony.” — editor's note: Gen. xx. 12.
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Clement of Alexandria, The Stromata, or Miscellanies VI.12Book VI
“And did not Abraham, when he was in danger on account of Sarah’s beauty, with the king of Egypt, properly call her sister, being of the same father, but not of the same mother?” — editor's note: The reading of the text has, “not of the same mother, much less of the same father,” which contradicts Gen. xx. 12, and has been therefore amended as above.
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