Rashi on Numbers 12:12/רש"י על במדבר י״ב:י״ב

1אל נא תהי LET NOT our sister BE —

2כמת AS ONE WHO IS DEAD — Aaron said this because a leper may be regarded as dead. How is it in the case of a corpse? It makes one unclean through entering a room in which it lies, So too, does the leper make a person unclean through his entering the place where he is (cf. Sifrei Bamidbar 105).

3אשר בצאתו מרחם אמו It ought to have stated “אמנו” “of our mother’s womb”, only that Scripture modified the expression. Similarly חצי בשרו — it ought to have said חצי בשרנו “the half of our flesh is consumed”, only that Scripture modified the expression (Scripture uses these expressions with the suffix of the third person singular instead of the first person plural, because it wishes to avoid an ominous expression referring to Aaron and Moses). — The meaning is: Since she came forth from the womb of our own mother she in her present state is to us as though the half of our flesh were consumed. It is the same idea as is expressed in the words, (Genesis 37:27) “for he is our brother, our own flesh” (Sifrei Bamidbar 105). — And even according to what the text literally implies (without assuming any modification) it appears to have that meaning: "It is not right for a brother to allow his sister to remain as a dead person".

4 אשר בצאתו — since he (the dead person) also come out of the womb of the mother of this man who possessed the power to help him and does not help him, — then, indeed, the half of his own flesh becomes consumed if the other perishes, since one’s brother is one’s own flesh. — Another explanation of אל נא תהי כמת in the sense of “Do not let her remain a leper” is: If you (Moses) do not heal her by your prayer, who will put her in quarantine as a leper (cf. Leviticus 13:4) who will ultimately declare her clean? It is impossible for me to examine her as to the character of her leprosy, since I am a near relative and a near relative may not examine the leprous plagues of his kin, (Sifrei Bamidbar 105; cf. Mishnah Negaim 2:5) and there is no other priest in The world who is not her relative. This is alluded to in the words אשר כצאתו מרחם אמו — “since he (the only person who could declare her clean) has come out of the same womb".