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.

I have written in the sexual prohibition of the mother (Sefer HaChinukh 190) about the root of the prohibition of relatives.

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.