…her son’s daughter… (Leviticus 18:17)
The previous mitzvah forbade sexual relations with a woman and her daughter. This one prohibits a woman and her granddaughter through a son. In other words, if a man marries the older woman, his step-granddaughter becomes prohibited. If he marries the younger woman, his grandmother-in-law becomes prohibited.
This prohibition ought not surprise us, as many of the arayos go up and down the family tree. Rabbinically, measures were enacted against relations with the mother of one’s father-in-law and other relatives who are conceptually similar.
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 #338 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #122 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.