וַיֹּ֣אמֶר מֹשֶׁ֣ה אֶֽל־אַהֲרֹ֡ן וּלְאֶלְעָזָר֩ וּלְאִֽיתָמָ֨ר ׀ בָּנָ֜יו רָֽאשֵׁיכֶ֥ם אַל־תִּפְרָ֣עוּ ׀ וּבִגְדֵיכֶ֤ם לֹֽא־תִפְרֹ֙מוּ֙ וְלֹ֣א תָמֻ֔תוּ וְעַ֥ל כׇּל־הָעֵדָ֖ה יִקְצֹ֑ף וַאֲחֵיכֶם֙ כׇּל־בֵּ֣ית יִשְׂרָאֵ֔ל יִבְכּוּ֙ אֶת־הַשְּׂרֵפָ֔ה אֲשֶׁ֖ר שָׂרַ֥ף יְהֹוָֽה׃
And Moses said to Aaron and to his sons Eleazar and Ithamar, “Do not bare your heads and do not rend your clothes, lest you die and anger strike the whole community. But your kin, all the house of Israel, shall bewail the burning that GOD has wrought.
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation.)
As the apposition “all the house of Israel” makes clear, here the plural noun אַחִים (literally “brothers”) denotes wider kinship or kin-like relations, as in Lev 10:4; 19:17; 25:25.
Unlike in contemporary English, this Hebrew term does not ascribe gender in and of itself. When a grammatically masculine referring expression categorizes (rather than identifies) its referent, the referential function is generic, and so is the referential gender. Women are in view by default. The proof is that when GOD makes a categorizing reference via even the singular ‘masculine’ form אָח, other characters matter-of-factly consider women to be included (Jer 34:14 in light of vv. 9, 16).
On how social gender is ascribed (or not) in Biblical Hebrew references, see Stein 2008; Stein 2013.
In terms of cultural norms, it likewise went without saying that women are in view in the present situation, because women were the face of public mourning in ancient Israel (Exod 33:4; Jer 9:16–17, 19; 2 Chron 35:25). The ancients could have hardly imagined that the Israelites camped in the wilderness might have “bewailed” the deaths of Nadab and Abihu without the involvement of women.
In short, there is no warrant for rendering in gendered terms.
As for translation into English, the NJPS rendering “kinsmen” nowadays tends to be taken as a male-only term. The revised rendering is more clearly gender-inclusive.