Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

The second case was when the temptation to idolatry proceeded from the nearest blood-relations and friends. The clause, "son of thy mother," is not intended to describe the brother as a step-brother, but simply to bring out the closeness of the fraternal relation; like the description of the wife as the wife of thy bosom, who lies in thy bosom, rests upon thy breast (as in Deu 28:54; Mic 7:5), and of the friend as "thy friend which is as thine own soul," i.e., whom thou lovest as much as thy life (cf. Sa1 18:1, Sa1 18:3). בּסּתר belongs to יסית: if the temptation occurred in secret, and therefore the fact might be hidden from others. The power of love and relationship, which flesh and blood find it hard to resist, is placed here in contrast with the supposed higher or divine authority of the seducers. As the persuasion was already very seductive, from the fact that it proceeded from the nearest blood-relations and most intimate friends, and was offered in secret, it might become still more so from the fact that it recommended the worship of a deity that had nothing in common with the forbidden idols of Canaan, and the worship of which, therefore, might appear of less consequence, or commend itself by the charm of peculiarity and novelty. To prevent this deceptive influence of sin, it is expressly added in Deu 13:8 (7), "of the gods nigh unto thee or far off from thee, from the one end of the earth even unto the other end of the earth," i.e., whatever gods there might be upon the whole circuit of the earth.