…or her daughter’s daughter, you shall not take in order to uncover… (Leviticus 18:17)
This mitzvah is essentially the same as the previous one, substituting “daughter’s daughter” for “son’s daughter.”
The upshot of this group of three mitzvos is that when a man marries a woman, six ancestors/descendants become forbidden to him by Torah law: his wife’s mother, her grandmother on her mother’s side, her grandmother on her father’s side, her daughter, a son’s daughter, and a daughter’s daughter. (Her sisters are likewise prohibited to her husband, as we’ll see in the next mitzvah, but we’re not talking about them right now.)
By rabbinic enactment, a man may also not engage in relations with a mother-in-law’s grandmothers, a father-in-law’s grandmothers, a stepson’s son’s daughter or a stepdaughter’s daughter’s daughter. They did not, however, place a ban on marrying a stepson’s daughter’s daughter or a stepdaughter’s son’s daughter, as these cases are not similar to the relationships forbidden by the Torah. (You may want to use a pencil and paper to diagram all these relationships!)
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractates of Yevamos (97a) and Sanhedrin (76a-b) and is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #339 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #123 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.