The text of the Book of Mormon hints at the idea that when Lehi's group arrived in the promised land, not only were there others there, but they were apparently soon included in Nephite society. According to John Sorenson,
within a few years Nephi reports that his people "began to prosper exceedingly, and to multiply in the land" (2 Nephi 5:13). When about fifteen years had passed, he says that Jacob and Joseph had been made priests and teachers "over the land of my people" (2 Nephi 5:26, 28). After another ten years, they "had already had wars and contentions" with the Lamanites (2 Nephi 5:34). After the Nephites had existed as an entity for about forty years (see Jacob 1:1), their men began "desiring many wives and concubines" (Jacob 1:15). How many descendants of the original party would there have been by that time? . . .
My reading of the text puts about eleven adults and thirteen children in Nephi's group when they split with the faction of Laman and Lemuel. However the adults included only three couples. None of the unmarried persons, including Nephi's brothers Jacob and Joseph and, probably, their sisters, would have had marriage partners available until nieces or nephews came of age. [John L. Sorenson, "When Lehi's Party Arrived, Did They Find Others in the Land?," in Journal of Book of Mormon Studies, Vol. 1/1, FARMS, Fall 1992, pp. 2-3]