Negative commandment · Lo Ta’aseh #339 of 613
Not to have intimate relations with a woman and her daughter's daughter
שלא לגלות ערוות אישה ובת בתה
The verse · Vayikra 18:17
עֶרְוַת אִשָּׁה וּבִתָּהּ לֹא תְגַלֵּה אֶת־בַּת־בְּנָהּ וְאֶת־בַּת־בִּתָּהּ לֹא תִקַּח לְגַלּוֹת עֶרְוָתָהּ שַׁאֲרָה הֵנָּה זִמָּה הִוא
“Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son’s daughter, or her daughter’s daughter, to uncover her nakedness; for they are her near kinswomen: it is foulness.”
The Rambam’s words
Not to have intimate relations with a woman and her daughter's daughter, as [Leviticus, ibid.] continues: "...her daughter's daughter,... you shall not take."
שלא לגלות ערוות אישה ובת בתה, שנאמר "ואת בת בתה, לא תיקח" (ויקרא יח,יז).
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה