DEUT622 R. Yochanan said in the name of R. Shimon b. Yehotzadak: Where is the prohibition against being alone with those who are sexually forbidden intimated in the Torah? "If there shall incite you your brother, the son of your mother." Now does the son of the mother incite, and not the son of the father? The verse, then, is an illusion to the halachah that a son may be alone with his mother, but [otherwise] it is forbidden to be alone with those who are sexually forbidden (Kiddushin 80b)