Leviticus 18:11

The nakedness of thy father's wife's daughter
Either the daughter of his father by another wife, which seems to be countenanced by what follows: begotten of thy father, she [is] thy sister;
but then this coincides with what is prohibited, ( Leviticus 18:9 ) , "the daughter of thy father"; that is, by another woman than a man's mother, only with this difference, that there is added, or "daughter of thy mother", that is, by another man than a man's own father; so that there is a prohibition of a sister whether by father or mother's side; here only as by the father's side, and so is only a part of that law; and, as some think, is for the confirmation of it, as Aben Ezra observes; or else the sense, as he thinks, is, that if a man marries a woman, and she has a little daughter by a former husband, that daughter may not be given in marriage to his son; and so the Septuagint version finishes this clause first, before it gives the other, which it considers as distinct from it, thus, "the shame of thy father's, wife's daughter thou shalt not uncover"; and then makes a distinct law of the latter; "she that is begotten of thy father is thy sister, thou shalt not uncover her shame"; but then this last falls in with ( Leviticus 18:9 ) , the Sadducees, as Aben Ezra also observes, by whom he means the Karaites, interpret it not of a mother's daughter, but of one brought up and educated by a man's father, and so is his adopted daughter, whom his son might not marry; and thus with the Romans it is said ^F7, that adoptive kindred hindered marriage between parents and children altogether; and among brethren so far forth as the loss of freedom did not intervene: some understand this law in this light, as De Dieu, that in ( Leviticus 18:9 ) ; the son of a second marriage is forbidden to marry with an half sister of the first marriage, whether she is the father's daughter, that is, which the father had by his deceased wife, or the mother's daughter, that is, which his mother had by a deceased husband; but here the son of a first marriage is forbidden with a half sister of a second marriage, which his mother-in-law has bore to his father, and is therefore called "the daughter of thy father's wife"; that is, of thy stepmother, but so the same may be said to be "begotten of thy father"; and therefore one begotten in a former marriage may not be understood; but then as this forbids the marriage of a brother with a sister, that is, of the same father, though not of the same mother, it falls in within the former law; wherefore some ^F8 have been of opinion, that this law forbids a man to marry the daughter of a woman whom his father has taken to wife, who was his deceased brother's wife, upon the law in ( Deuteronomy 25:5 ) ; by which marriage she became the father's daughter, and the son's sister; wherefore they take the phrase, "begotten of thy father", to signify "being akin" to thy father; which, if it can be established, makes a distinct law: Jarchi observes, on this phrase, "the daughter of thy father's wife",

``this teaches that a man is not guilty concerning his sister that is by an handmaid or stranger; therefore it is said, the daughter of thy father's wife, namely, one that was fit for marriage.''