14 These be the heads of their fathers’ houses: The sons of Reuben the firstborn of Israel; Hanoch, and Pallu, Hezron, and Carmi: these be the families of Reuben. 15 And the sons of Simeon; Jemuel, and Jamin, and Ohad, and Jachin, and Zohar, and Shaul the son of a Canaanitish woman: these are the families of Simeon.
The account in Exodus 6 suddenly pivots to describing the clans that grew out of the twelve tribes of Israel. It begins with the main households of Reuben, the firstborn, and Simeon, the second-born. The ancestry of these two households does not go past the second generation, though, and there were certainly other generations before that of the current narrative.
But, take notice, the record is not actually trying to give us a full genealogy here. As we will see tomorrow, after Reuben and Simeon are briefly mentioned, the record goes into much fuller detail with the descendants of Levi. And after doing that, none of the descendants of the other tribes of Israel will be detailed at all!
Therefore, this is not a standard genealogy sequence in the Bible. Presumably Reuben and Simeon are only mentioned to establish context before diving into the details of the third-born son Levi. These and the following verses are actually just helping us to understand the link from Levi to Aaron and Moses, and the eventual transformation of that tribe into the priest class.