As a part of the process that is reported in this chapter of the assembling of the Nephites and the returning groups of Zeniff and Alma, these new arrivals could choose to be numbered among the Nephites. They were baptized and organized into units within the church of God. This step of joining, assembling, renaming, numbering, and belonging created for them, as it does in the Church today, a sense of extended family, of being brothers and sisters, of working together, caring for one another, all as a result of being adopted as sons and daughters of Christ.
Notice in verse 12 that even the children of Amulon and his brethren, the priests of Noah who had captured and married the daughters of the Lamanites, were displeased at the behavior of their fathers, and did not want to be identified any longer by the name of their fathers: "Therefore they took upon themselves the name of Nephi, that they might be called the children of Nephi and be numbered among those who were called Nephites." This social and spiritual practice, in which people who were not closely related to each other claim to have a family connection, is known by anthropologists as "fictive kinship," and it was common in the ancient world, especially in religious contexts.
There are several examples of it in the Old Testament. When the Israelites left Egypt, they were referred to as a "mixed multitude" (Exodus 12:38), suggesting that all emigrants were not of the tribes of Israel. These newcomers were adopted into one of the twelve tribes of Israel, even though they were not technically part of these extended family groups. Each of these tribes was named after one of the sons of Jacob or Joseph, the great patriarchs of the Old Testament, although later, Joseph’s two sons, Ephraim and Manasseh, were given an inheritance with Jacob’s sons, as noted in Genesis 48:5.
This practice is evident throughout the Book of Mormon. Family was so important that those who belonged to a group that no longer had the same values, often identified themselves as Nephites. See, for example, Jacob 1:13–14, in which Jacob grouped the Nephites, Jacobites, Josephites, Zoramites, Lamanites, Lemuelites, and Ishmaelites into values-oriented tribes or fictive kinship groups: "Those who are friendly to Nephi, I shall call Nephites, or the people of Nephi." Those who sought to destroy the people of Nephi, Jacob said, "I shall call them Lamanites."
The importance of family relationships, whether by descendancy or adoption, is demonstrated throughout the Book of Mormon to be a primary human need fulfilled within the network of the Gospel of Jesus Christ and our always-loving Heavenly Father.
Book of Mormon Central, "What Does the Book of Mormon Teach about Families? (Mosiah 25:12)," KnoWhy 382 (November 16, 2017). "For many people, family is the most important thing in life. The same can likely be said for the peoples of the Book of Mormon. Despite their sometimes-complex origins, they organized themselves into several core family relationships, just like many other societies in the ancient world."
John L. Sorenson, "When Lehi’s Party Arrived in the Land, Did They Find Others There?" Journal of Book of Mormon Studies 1, no. 1 (1992): 1–34.