פּירוש אויף דברים 13:7

כי יסיתך IF [THY BROTHER …] ENTICE THEE — The term יסת always denotes to “stir up”, “to incite”, as it is said, (I Samuel 26:19) ”If it be the Lord that hath stirred thee up (הסיתך) against me” (Sifrei Devarim 87:2); inciter in O. F., Engl, to incite — i.e. that he induces one to act thus.

אחיך בן אמך, “your brother, the son of your mother.” The same applies to “your brother, the son of your father;” the reason why the Torah chose as its example the brother, son of your mother,” is that normally the bonds to one’s mother are closer, and one is in the same house with one’s mother all the time, while the father is in the field or elsewhere outside the house, earning his livelihood. Moreover, brotherly love to one another is often tainted by the fact that one has to share the father’s inheritance with one’s brother of the father, but not with a brother of the same mother.

כי יסתך אחיך בן אמך, “If your brother, son of your mother, will attempt to entice you (to worship an idol);” The reason that the Torah adds the words: “son of your mother,” here is that it is likely that brothers of the same mother, i.e. the same womb, are closer to one another than brothers who only share the same father. Furthermore, sons of only the same father are likely to be hostile to one another for reasons of the eventual inheritance.