TT #129 · Parashas Reeh · Third Aliyah · Passuk 13:7
If your brother, the son of your mother, or your son or your daughter or the wife of your bosom or your friend who is as your own soul, entices you in secret, saying, “Let us go and serve other gods” that you have not known, you or your fathers.
Torah Temimah points to Sanhedrin.
English: “Your brother, the son of your mother” — Does a mother’s son entice while a father’s son does not? Abaye said: The verse states the less obvious case. It is not necessary to mention the son of the father, who hates him and gives him bad counsel; but even the son of the mother, who does not hate him, one might have thought he would listen to him — therefore the verse teaches that even he is included.
Derash on the verse (Sanhedrin) — the choice of “son of your mother” teaches that even a trusted relative is included in the law of the mesis.
Even a beloved relative such as a maternal brother is included in the prohibition of the mesis; the law is not limited to hostile relatives.