Lévitique 18:7

The nakedness of thy father, or the nakedness of thy mother, thou shall not dishonour. A woman shall not lie with her father, nor a man with his mother; she is thy mother: thou shalt not discover her nakedness.

your father Scripture begins with the father (because the father precedes the son), and forbids a son from having relations with anyone who is a near kin of his father or mother. This primarily denotes his mother, and secondarily denotes his father’s wife even if she is not his mother. Afterward scripture lists his sister, whether she is his father’s daughter or his mother’s daughter.

THE NAKEDNESS OF THY FATHER, AND THE NAKEDNESS OF THY MOTHER, SHALT THOU NOT UNCOVER. Rashi commented: “The nakedness of thy father — this refers to thy father’s wife. And the nakedness of thy mother — this is intended to include one’s own mother, although she be not the wife of one’s father. The nakedness of thy father’s wife shalt thou not uncover — this is intended to include in this prohibition one’s father’s wife even after [the father’s] death.” Similarly Rashi explained: “Thou shalt not uncover the nakedness of thy father’s brother. And what does this [uncovering of his] nakedness mean? Thou shalt not approach to his wife.” This is indeed true according to the correct sense [of the verses]. But in the Gemara of Tractate Sanhedrin the Rabbis concluded that the verses, the nakedness of thy father, and the nakedness of thy father’s brother, are prohibitions against carnal relationships with these males themselves [and not with their wives], and [are required] to make the offender liable to a double penalty if he committed it in error [lying carnally with his father or uncle]. But I wonder, according to the opinion of the Sages [who explain the verse, the nakedness of thy father as being a prohibition against lying carnally with one’s father], why did He not likewise admonish against lying carnally with a son or brother, in addition to prohibiting [sexual relationships with their] wives? Perhaps the Torah was more particular in guarding the honor of the elders. However, each and every verse in this section [of forbidden relationships] constitutes only one prohibition, does not constitute two separate prohibitions as Rashi explained it [one referring to one’s father’s wife, and one to one’s own mother although she is not the wife of one’s father, as stated above], but they are one prohibition, as explained further on in the text. there being here no case of two admonitions in one prohibition, for the command concerning sisters [in Verse 9] and also daughters [in Verse 10] each constitute but one prohibition [and not, as Rashi explained there, that each verse contains two prohibitions]. The correct interpretation by way of the simple meaning of Scripture appears to me to be that the nakedness of thy father, and the nakedness of thy mother [shalt thou not uncover], constitute but one negative commandment and one admonition, which forbids intercourse with one’s own mother, for in having such intercourse he thereby uncovers the nakedness of his father and that of his mother. This is the sense of the [concluding] expression in this verse, she is thy mother, meaning that you have done with her two evils, in that you have uncovered the nakedness of your mother and that of your father. Following the admonition against uncovering the nakedness of one’s father, which refers [as explained] to one’s mother, He went back [in the following Verse 8] and admonished against [uncovering] the nakedness of one’s father’s wife, although she be not one’s mother, saying, The nakedness of thy father’s wife shalt thou not uncover; it is thy father’s nakedness. Similarly, Thou shalt not uncover the nakedness of thy father’s brother means [as the verse itself continues], thou shalt not approach to his wife. This is why it does not say “and” to [his wife thou shalt not approach].