TT #128 · 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 Kiddushin / Sanhedrin.
Sefaria — Kiddushin / Sanhedrin
English: “Your brother, the son of your mother” — Rabbi Yochanan said in the name of Rabbi Shimon ben Yehotzadak: This is a scriptural allusion to the prohibition of yichud, as it is written “If your brother, the son of your mother, entices you.” Does a mother’s son entice while a father’s son does not? Rather, it teaches that a son may be alone with his mother, but it is forbidden to be alone with any of the arayos of the Torah.
Remez / derash (Kiddushin) — deriving the Torah-level prohibition of yichud from the wording of the mesis verse.
There is a Torah-level allusion to the prohibition of yichud: a son may be alone with his mother, but one may not be alone with any of the arayos.