עַל־כֵּן֙ יַֽעֲזָב־אִ֔ישׁ אֶת־אָבִ֖יו וְאֶת־אִמּ֑וֹ וְדָבַ֣ק בְּאִשְׁתּ֔וֹ וְהָי֖וּ לְבָשָׂ֥ר אֶחָֽד׃
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Da'at Zekenim
כי אם לשארו הקרוב אליו, “except for his kin that is close to him;” according to Torat Kohanim, the word הקרוב אליו, excludes a girl only betrothed to him who has not yet become his wife in the full meaning of the word. Seeing that the priest may not defile himself on account of a woman only betrothed to him, it is clear that he may defile himself on account of a sister who is betrothed but not married yet. Concerning the words הקרובה אליו, Rashi explains that they include a sister who is betrothed. This makes sense, as seeing that an ordinary priest may not defile himself in order to bury a woman betrothed to him, it is logical that he should be allowed to defile himself for a sister who is only betrothed. If his sister’s husband to be had been a priest, he would not have been permitted to busy herself with her funeral. The Talmud in tractate Yevamot folio 22, understands the word שארו as referring to the priest’s properly married wife. This is also in agreement of what is written in verse 4: לא יטמא בעל בעמיו להחלו, “a husband shall not defile himself for his wife’s corpse if she had not been legally permitted to be the wife of a priest.” Unless the clause להחלו, “to disgrace him,” applied to his “wife,” he would, of course, be permitted to defile himself on her corpse. Furthermore, the fact that the Torah makes a distinction between a sister who has remained a virgin, (verse three) and it describes such a sister as הקרובה אליו, “who is close to him seeing that she had not been married,” by adding: “he may defile himself on her account,” it is clear that if she had been married already, her brother the priest, would not be permitted to defile himself at her funeral. The reason is simply that in that event her husband is considered as closer to her than her brother, though the former is a blood relative. The degree of relationship through marriage has been spelled out in the Torah already in Genesis 2,24, where the Torah described the bond of marriage as making “one flesh” out of husband and wife. There is no greater degree of קרובה, physical intimacy, than that.
Radak
והיא בעלת בעל, she is out of bounds to you. Even the first human being had already been commanded not to commit adultery, as our sages derived from the words ודבק באשתו, meaning that he is to have intimate relations exclusively with his own wife. (Genesis 2,24) From all we read it is clear that the people of that time considered the sin of adultery so severe that they were prepared to kill the husband in order not to violate the commandment not to commit adultery. (compare the comment made by Avraham to Sarah in 12,12) They were aware of the sin of committing murder but considered it as a lesser sin. Our sages paid attention to the expression בעלת בעל as opposed to אשת איש, suggesting that according to Avimelech’s mores, a woman who had become married but whose marriage had not yet been consummated would not be included in the prohibition of being someone else’s wife. Girls who were merely betrothed would not be considered as out of bounds on account of their being betrothed. [a major difference between Jewish law and the Noachide laws. Ed.]
Rav Hirsch
V. 4. דבק ,ואתם הדבקים ist der innigste Anschluss an einen andern, ודבק באשתו (Bereschit 2, 24 – siehe daselbst 9, 21), der sich somit nicht der leisesten Abneigung bewusst ist. עיניכם הראות heißt es im vorigen Verse, euch liegt die Selbsterfahrung vor Augen. Es ist keiner unter euch, der nicht weiß, daß er sich auch nicht die leiseste Abweichung zum Peor hin hat zu Schulden kommen lassen und darum mitten in der Pest, die die Schuldigen hinraffte, erhalten blieb. Dadurch hat jeder an sich selbst den stärksten Beweis von dem Werte des Gottgehorsams und die stärkste Mahnung zu wandelloser Treue. Diese Erfahrung machte die Gesamtheit in allen ihren Gliedern. Darum der Plural: ואתם וגו׳ ,עיניכם וגו׳. Die strafende Vernichtung traf aber die Schuldigen mit Rücksicht auf die ihrer reinen Gottesbestimmung einheitlich zu erhaltende Nation, daher Singular: מקרבך. Die Bestimmung Israels als Gottesvolk duldet keine Peorsjünger in seiner Mitte.