The term Lamanite is used in two ways in the Book of Mormon. It is Jacob himself who describes the dual meaning of Lamanite and Nephite as both lineage names and collective cultural names (Jacob 1:13). In this verse, the probable reading is lineage, specifically because Jacob gives a descriptive addition: "the Lamanites, our brethren." The Lamanites who share an inherited culture with the Nephites are keeping this commandment (although they have broken many others). Jacob will return to this contrast of Lamanite and Nephite in the next chapter in the current text, although it is a continuation of this same sermon.
Redaction: It is important to remember that while there is chapter break in our current versions of the Book of Mormon, there was no chapter division here in the 1830 edition. This is important because we have in our current Jacob 2 an 3 a single discourse given at a specific location without any conceptual division. Had there been a division in the text on the plates themselves, we would be required to explain that division. In this case the division is artificial, unlike the division between 2 Nephi 31 and 32 which did exist on the plates, and therefore did require an explanation or the reason a division existed.