Powtórzonego Prawa 7:4 Komentarz: Rashi, Chizkuni, Ibn Ezra & Rabbeinu Bahya

כי יסיר את בנך מאחרי FOR HE WILL TURN AWAY THY SON FROM FOLLOWING ME — i.e., the son of the heathen when he marries thy daughter, will turn away thy son (grandson) whom thy daughter will bear unto him from following Me. This teaches us, that thy daughter's son who is born of a heathen is termed thy son, but thy son’s son, who is born of a heathen woman, is not termed thy son, but her son, for, you see, in regard to the statement: “his daughter thou shalt not take [to thy son]" it does not add “For she will turn away thy son (grandson) from following Me" (Yevamot 23a).

כי יסיר, “for he will turn away;” this is the son of a Canaanite marrying a Jewish girl, mentioned in verse three. The father of such a child will bring him up as a heathen, although legally, he is a Jew, his mother being Jewish. In the case of the child being a daughter, this will be even more so, as living as a heathen is so much easier for the children who do not have to observe all the commandments of the Jewish religion. (B’chor shor)

will turn your son away The subject of the sentence is “his son” mentioned above [: 3] — his son to whom you should not give your daughter in marriage or The subject of the sentence is your foreign neighbor, whose daughter you should not take for your son in marriage, as mentioned above