Halakhah for Yevamot 42:20

From the laws of the commandment is, for example, that which they, may their memory be blessed, forbade (Yevamot 21b) - to make a fence for this - the daughter-in-law of his son and likewise the daughter-in-law of the son of his son, to the end of the world. And they also forbade only the daughter-in-law of his daughter. And the reason is similar to that which we said above - since the prohibition came in the verse about his daughter-in-law, they decreed about all daughters-in-law that come on account of the son; and they decreed with [the daughter-in-law of] the daughter only, and that is enough for us with that. And these are called secondary [prohibitions].