Jacob and Joseph becoming priests does not refer to our present Aaronic Priesthood offices. President Joseph Fielding Smith explains:
There were no Levites who accompanied Lehi to the Western Hemisphere. Under these conditions the Nephites officiated by virtue of the Melchizedek Priesthood from the days of Lehi to the days of the appearance of our Savior among them. It is true that Nephi “consecrated Jacob and Joseph” that they should be priests and teachers over the land of the Nephites, but the fact that plural terms priests and teachers were used indicates that this was not a reference to the definite office in the priesthood in either case, but it was a general assignment to teach, direct, and admonish the people. Otherwise the terms priest and teacher would have been given, in the singular.
Living after the manner of happiness (v. 27) is probably Nephi’s way of saying they were basically following the commandments of God. As quoted and discussed in Chapter Six of this work, the Prophet Joseph taught that happiness comes from following the path of “virtue, uprightness, faithfulness, holiness, and keeping all of the commandments of God” (TPJS, 255–256).