כי יסיתך אחיך, "If your brother entices you, etc." In this paragraph the Torah teaches that enticement to sin is not only due to external sources but may be due to something within us, in fact some force born within us from the day we see the light of day. Our sages in Sanhedrin 91 derive this from Genesis 4,7: "sin rests at the door."
There is an additional internal "enticer," i.e. the spiritually negative force man creates every time he sins. This is the mystical dimension of Avot 4,2 that "one sin drags another in its wake." Kabbalists have talked at length about this subject explaining that every sin creates an accuser which in turn increases the power of the original evil urge within us. The more man sins the more powerful the forces which urge him to sin become. This is the meaning of Berachot 34 that מקום שבעלי תשובה עומדים אין צדיקים גמורים יכולין לעמוד, "that it is not possible to draw a valid comparison between the forces of evil to which a repentant sinner is exposed and to similar forces to which the person who is totally without sin is exposed."
There is a third dimension to the personality of a person who indulges in sin. By doing so he acquires an additional soul, as it were. Just as we have a principle that he who observes the Sabbath is granted an additional soul (spiritually positive force) on that day, the reverse holds true for the habitual sinner.
When you continue on such an evil path, allowing yourself to be enticed into sin, one eventually integrates one's personality with the forces of evil so much so that instead of merely becoming the victim of enticement one denies the essence of Judaism leading one to embrace idolatry.
When Moses speaks of בי יסיתך אחיך, "when your brother entices you," he describes the evil urge as someone who considers himself as "your brother" from birth. He adds the words בן אמך, "son of your mother," to explain whence the enticer originates, i.e. he is a "son" of our common mother Eve who was responsible for the fact that good and evil are inextricably mixed within us at birth. Concerning the second aspect of evil within us, i.e. the one created by our own sins -as opposed to Eve's- Moses says או בנך או בתך, "or your own son or your own daughter." These are references to the spiritually negative creations which are the result of our own sins. They too are called alien children. The reason that the Torah lists both בנך, "your son," as well as בתך, "your daughter" to describe such creations, is that some sins produce accusers of a masculine type whereas other sins produce accusers of a feminine variety. It all depends on the type of sin one has committed. The greater the sin the more likely that the accuser it creates is of the masculine type. If the sin is relatively minor, the accuser it creates may be described as בתך, "your daughter."
With regard to the third acquisition i.e. an evil soul which hence-forth accompanies the sinner constantly, Moses describes it as אשת חיקך, "the wife of your bosom," as it now resides inside the sinner and will add to the power of the forces which entice man to sin.
Regarding the fourth element which entices man to sin, the opposite of the נשמה יתירה, the "additional soul" a Sabbath-observing Jew is equipped with on the Sabbath, the Torah calls it רעך אשר כנפשך, "your friend who is like your own soul." Once the sinner has been "softened up" by the enticer, the Torah describes the nature of the enticement, i.e. an invitation to serve alien gods.
Moses makes a point of describing this enticement as occurring בסתר, "secretly," seeing the enticer is in the habit of hiding his true purpose pretending to love you and to have your true interests at heart.
לאמור נלכה, saying "let us go." Moses inserts the word לאמור to make it clear that the enticer does not openly use words such as "let us commit idolatry," but he expresses himself more obliquely; all the while he aims at the eventual objective, i.e. to commit idolatry. When Moses portrays the enticer saying נלכה נעבוד, "let us go and serve, etc," he includes himself in such a way that his victim thinks that he too had never previously served idols. This makes the victim think that his own soul is speaking to him rather than an outside force which he would define as the evil urge against which he would be on guard.
אשר לא ידעת, "which you have never known, etc." Moses describes the enticer as marshalling arguments against the undertaking by the Israelites at the time of the revelation not to serve any alien deities. He argues that the undertaking entered into by the Israelites at that time included only the religions which were known at that time. He argues that any religion, i.e. form of idolatry, which did not exist at that time or which the Israelites did not know of at that time, was not something they were committed not to serve. He emphasizes his point by stressing אשר לא ידעת אתה ואבותיך, "which neither you personally nor your ancestors were aware of." When your forefathers accepted the prohibition not to serve idols, these idols were not included either in what they undertook not to serve.