John Calvin Commentary Numbers 26:57

"And these are they that were numbered of the Levites after their families: of Gershon, the family of the Gershonites; of Kohath, the family of the Kohathites; of Merari, the family of the Merarites." — Numbers 26:57 (ASV)

And these are they that were numbered of the Levites. He treats separately of the tribe of Levi, which God had set apart from the rest of the people. Among the sons of Levi, the last one mentioned is Kohath, the founder of the priestly family.

From this, we may probably conjecture that the law of primogeniture was not regarded when God chose to take the priests from them.

But why Moses would expressly state the name of his mother, contrary to the usual custom of Scripture, does not clearly appear. It is not likely that he did this as a distinction for his own family, because he simultaneously shows how he himself, as well as his children, was deprived of the honor (of the priesthood).¹⁹⁶ In this, there is certainly no appearance of ambition.

It is more probable, if the word daughter is taken literally, that he did not conceal a disgraceful circumstance in order to more highly extol the indulgence of God. For, in this case, Moses and Aaron were born of an incestuous marriage, since Amram, their father, must have married his aunt—a union that natural modesty forbade. It will, then, be a rather candid confession of family dishonor, rather than an ambitious boast.

If we inquire how this could have been tolerated, the answer readily suggests itself: this permissiveness had so largely prevailed among the Eastern nations that no one considered illicit what was in such universal use. And this we shall soon see¹⁹⁷ to be expressly referred to when God, by forbidding incestuous marriages, distinguishes His people from other nations. It will be no matter of surprise, then, that those who were not yet prohibited from doing so by the law of God had followed the general custom.

¹⁹⁶ Added from Fr.

¹⁹⁷ The Fr. more correctly says, “Ce que nous avons veu ci dessus;” this we have seen above; — the table of prohibited degrees having been considered . more correctly says, “Ce que nous avons veu ci dessus;” this we have seen above; — the table of prohibited degrees having been considered ante, vol. 3, p. 96, vol. 3, p. 96, et seq.