The nakedness of a woman and the daughter of her son: To not reveal the nakedness of a woman and the daughter of her son. And we have already said that this prohibition is always with marriage to the first one (as a condition), as it is stated (Leviticus 18:17), “the daughter of her son [...] you shall not take.”

From the laws of the commandment is that which they, may their memory be blessed, forbade (Yevamot 21a) the daughter of the son of her son as a fence for this — and it is included in the secondary [prohibitions]. As everything the prohibition of which is from the words of the [Rabbis] is called a secondary prohibition. And so [too,] did they forbid only the mother of the mother of the father of his wife.

And this prohibition is practiced in every place and at all times. And [in the case of] one who transgresses it and volitionally laid [with] a woman and the daughter of her son, with the marriage of the first one — in the lifetime of the first — both of them are burned; but if the first one died, both of them are [punished] with excision. If it was inadvertent, they bring a fixed sin-offering. And if he laid with the secondary [prohibition], he is struck with rabbinic lashes of rebellion.