Deuteronomy 23:1

לא יקח — This does not mean “he shall not take” but “he cannot take” his father’s wife: there can be no question of a legal marriage for him in regard to her, because the marriage ceremony (קדושין) has no legal hold on her (cannot make her his wife: it is no marriage) (Kiddushin 67b; cf. Rashi on Kiddushin 67b s. v. לא יקח) ולא יגלה כנף אביו AND HE SHALL NOT UNCOVER HIS FATHER’S SKIRT — This refers to the שומרת יבם of his father (the widow of his father’s brother who died without issue, and who is waiting (שומרת) for her brother-in-law (יבם) either to marry her or to put her through the ceremony of release, חליצה), who is thus destined for his father. But has he not already been prohibited about her (i.e. forbidden to marry her) on account of the law (Leviticus 18:14) “the nakedness of thy father’s brother [thou shalt not uncover]”?! But the prohibition is repeated here in order to make him transgress two negative commands if he takes her (Yevamot 4a), and in order to put into juxtaposition to it the law (v. 2) “one born of incest or adultery (ממזר) shall not come [into the assembly of the Lord]”, and thereby to teach that one is termed ממזר only if he is born from those liable to the penalty of excision on account of the intercourse between them, as is the case with one who take’s his father’s שומרת יבם, who is forbidden to him under the penalty of כרת as אשת אחי אביו; cf. Leviticus 18:14 and Leviticus 18:29 (but not if he was born of a woman intercourse with whom involves only flagellation), and it logically follows that the term applies also to one born from those liable to one of the death penalties by sentence of the court, for amongst the cases of forbidden intercourse there is none punishable with death by the sentence of the court which does not involve the penalty of excision (if it was not preceded by a warning) (Yevamot 49a; cf. also Rashi on Kiddushin 67b s. v. מהנ״מ).

לֹא־יִקַּ֥ח אִ֖ישׁ אֶת־אֵ֣שֶׁת אָבִ֑יו וְלֹ֥א יְגַלֶּ֖ה כְּנַ֥ף אָבִֽיו׃ {ס}        

A man shall not take his father’s wife, nor uncover his father’s skirt.