וּבְנֵי אֱלִיאָב נְמוּאֵל וְדָתָן וַאֲבִירָם הוּא דָתָן וַאֲבִירָם
קְרִיאֵי (כתיב קרואי) הָעֵדָה אֲשֶׁר הִצּוּ עַל משֶׁה וְעַל אַהֲרֹן
בַּעֲדַת קֹרַח בְּהַצֹּתָם עַל יְהֹוָה:
The sons of Eliab were Nemuel, Dathan and Abiram they are Dathan and
Abiram, the chosen of the congregation who incited against Moses and
Aaron in the assembly of Korah, when they incited against the Lord.
Note the translation of הִצּוּ as incited. That is Dathan and Aviram were the active leaders in the tribe of Reuven of their part of the rebellion. They would have rebelled against Korach (had he won) since they were against anyone else becoming the leader. Remember that they were inciting against Moshe from the beginning and the medrash says that they were the ones who informed against Moshe about the Egyptian task master that he had killed.
Nemuel, had he been part of the rebellion, would not have been mentioned, since he would have been like anyone else in the tribe of Reuven. Additionally, we do not know if he took part in that rebellion or not as only the inciters (Dathan and Aviram) are explicitly mentioned.
Note that only Korach is mentioned of the 250 who died because of the incense because he was the instigator of the rebellion.
Rav Hirsch says
According to Baba Basra 117b, all those that took part in the
rebellion of Korach were considered as non-existant in the division of
the land whereas the other יוצאי מצרים even if they had died before
the entry in the land had to be considered as the real people taking
possession of the land amongst whom the land was to be divided.
According to Ramban this is the reason for mentioning the death of
Datahn and Aviram here.