To not reveal the nakedness of the sister of the father: To not have intercourse with the sister of the father, as it is stated (Leviticus 18:12), “The nakedness of the sister of your father you shall not reveal, etc.”
From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Yevamot 54b) that the prohibition of the sister of the father is whether she is the sister of his father from the father only or from the mother only; whether she is the sister from marriage or whether from licentiousness — as each one of these is called his sister regardless. And there is no room for secondary [prohibitions] here.
And this prohibition is practiced in all places and at all times. And one who transgresses it and lays with the sister of his father volitionally is liable excision; inadvertently, he brings a fixed sin-offering.
שֶׁלֹּא לְגַלּוֹת עֶרְוַת אֲחוֹת אָב – שֶׁלֹּא לָבוֹא עַל אֲחוֹת הָאָב, שֶׁנֶּאֱמַר (ויקרא יח יב) עֶרְוַת אֲחוֹת אָבִיךָ לֹא תְגַלֵּה וְגוֹ'.
To not reveal the nakedness of the sister of the father: To not have intercourse with the sister of the father, as it is stated (Leviticus 18:12), “The nakedness of the sister of your father you shall not reveal, etc.”
מִשָּׁרְשֵׁי הָעֲרָיוֹת כָּתַבְנוּ מָה שֶׁיָּדַעְנוּ (לעיל קצ).
We have written what we know of the roots of sexual prohibitions (Sefer HaChinukh 190).
מִדִּינֵי הַמִּצְוָה. כְּגוֹן מַה שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה (יבמות נד, ב) שֶׁאִסּוּר אֲחוֹת אָב, בֵּין שֶׁהִיא אֲחוֹת לְאָבִיו מִן הָאָב בִּלְבַד אוֹ מִן הָאֵם לְבַד, וּבֵין שֶׁהִיא אֲחוֹתוֹ מִן הַנִּשּׂוּאִין בֵּין מִן הַזְּנוּת, שֶׁכָּל אַחַת מֵאֵלּוּ מִכָּל מָקוֹם אֲחוֹת אָבִיו נִקְרֵאת. וְאֵין כָּאן מָקוֹם לִשְׁנִיּוֹת.
From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Yevamot 54b) that the prohibition of the sister of the father is whether she is the sister of his father from the father only or from the mother only; whether she is the sister from marriage or whether from licentiousness — as each one of these is called his sister regardless. And there is no room for secondary [prohibitions] here.
דִּינֵי הַמִּצְוָה. בְּסוֹף הַסֵּדֶר תִּרְאֶה מְקוֹמָן [א"ה שם].
You will see the place of the laws of the commandment at the end of the Order. (See Tur, Even HaEzer 21.)
וְנוֹהֵג אִסּוּר זֶה בְּכָל מָקוֹם וּבְכָל זְמַן. וְהָעוֹבֵר עָלֶיהָ וְשָׁכַב אֶת אֲחוֹת אָבִיו בְּמֵזִיד חַיָּב כָּרֵת, בְּשׁוֹגֵג מֵבִיא חַטָּאת קְבוּעָה.
And this prohibition is practiced in all places and at all times. And one who transgresses it and lays with the sister of his father volitionally is liable excision; inadvertently, he brings a fixed sin-offering.