To not reveal the nakedness of the daughter of the son: To not reveal the nakedness of the daughter of the son, as it is stated (Leviticus 18:10), “The nakedness of the daughter of your son, etc. do not reveal.” And according to what appears, the prohibition is whether the son is fit or even if he is a mamzer — he is his son regardless — and whether the daughter of the son is fit or even if she is a mamzeret.
From the laws of the commandment is that which they, may their memory be blessed, forbade (Yevamot 22a) as a fence for this prohibition — and that is just the daughter of the daughter of his son and, likewise, just the daughter of the son of his son. And some say that there is no end to these prohibitions ever, since they forbade a man all that “come out from his thigh” for all generations. And you will see where the rest of its details are at the end of the Order (Sefer HaChinukh 211). (See Even HaEzer 21.)
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with the daughter of his son volitionally — and there are witnesses — is liable for burning, as they, may their memory be blessed, said in Sanhedrin 75a; and if there are no witnesses, he is liable for excision; inadvertently, he brings a fixed sin-offering at the time of the [Temple]. And if he laid with the daughter of the daughter of the son and all the other secondary ones, he is struck with lashes of rebellion.
שֶׁלֹּא לְגַלּוֹת עֶרְוַת בַּת הַבֵּן – שֶׁלֹּא לְגַלּוֹת עֶרְוַת בַּת הַבֵּן, שֶׁנֶּאֱמַר (ויקרא יח י) עֶרְוַת בַּת בִּנְךָ וְגוֹ' לֹא תְגַלֶּה. וּלְפִי הַדּוֹמֶה כִּי הָאִסּוּר הוּא בֵּין שֶׁהַבֵּן כָּשֵׁר אוֹ אֲפִלּוּ מַמְזֵר בְּנוֹ הוּא מִכָּל מָקוֹם, וּבֵין שֶׁבַּת הַבֵּן גַּם כֵּן כְּשֵׁרָה אוֹ אֲפִלּוּ מַמְזֶרֶת.
To not reveal the nakedness of the daughter of the son: To not reveal the nakedness of the daughter of the son, as it is stated (Leviticus 18:10), “The nakedness of the daughter of your son, etc. do not reveal.” And according to what appears, the prohibition is whether the son is fit or even if he is a mamzer — he is his son regardless — and whether the daughter of the son is fit or even if she is a mamzeret.
שֹׁרֶשׁ אִסּוּר הַקְּרוֹבוֹת, כְּתַבְתִּיהוּ בְּאִסּוּר עֶרְוַת אֵם (מצוה קצ).
I have written in the sexual prohibition of the mother (Sefer HaChinukh 190) about the root of the prohibition of relatives.
מִדִּינֵי הַמִּצְוָה. מָה שֶׁאָסְרוּ זִכְרוֹנָם לִבְרָכָה (יבמות כב, א) לְגֶדֶר אִסּוּר זֶה, וְהִיא בַּת בַּת בְּנוֹ בִּלְבַד, וְכֵן בַּת בֶּן בְּנוֹ בִּלְבַד. וְיֵשׁ אוֹמְרִים, שֶׁאֵין הֶפְסֵק לְאִסּוּרִים אֵלּוּ לְעוֹלָם כִּי כָּל יוֹצְאֵי יְרֵכוֹ שֶׁל אָדָם אָסְרוּ עָלָיו עַד סוֹף כָּל הַדּוֹרוֹת. וְיֶתֶר פְּרָטֶיהָ, בְּסוֹף הַסֵּדֶר (מצוה ריא) תִּרְאֶה הֵיכָן הֵם [א"ה שם].
From the laws of the commandment is that which they, may their memory be blessed, forbade (Yevamot 22a) as a fence for this prohibition — and that is just the daughter of the daughter of his son and, likewise, just the daughter of the son of his son. And some say that there is no end to these prohibitions ever, since they forbade a man all that “come out from his thigh” for all generations. And you will see where the rest of its details are at the end of the Order (Sefer HaChinukh 211). (See Even HaEzer 21.)
וְנוֹהֵג אִסּוּר זֶה בְּכָל מָקוֹם וּבְכָל זְמַן. וְעוֹבֵר עָלֶיהָ וְשָׁכַב אֶת בַּת בְּנוֹ בְּמֵזִיד וְיֵשׁ עֵדִים חַיָּב שְׂרֵפָה, כְּמוֹ שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה בְּסַנְהֶדְרִין (עה, א) וְאִם אֵין עֵדִים חַיָּב כָּרֵת, בְּשׁוֹגֵג מֵבִיא חַטָּאת קְבוּעָה בִּזְמַן הַבַּיִת, וְאִם שָׁכַב אֶת בַּת בַּת בְּנוֹ וְכֵן כָּל שְׁאָר הַשְּׁנִיּוֹת לוֹקֶה מַכַּת מַרְדּוּת.
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with the daughter of his son volitionally — and there are witnesses — is liable for burning, as they, may their memory be blessed, said in Sanhedrin 75a; and if there are no witnesses, he is liable for excision; inadvertently, he brings a fixed sin-offering at the time of the [Temple]. And if he laid with the daughter of the daughter of the son and all the other secondary ones, he is struck with lashes of rebellion.