Doubtless, the Nephites were guilty of religious ethno-centrism. They had become a little self-righteous. Jacob pulls down their pride by explaining that they are more wicked than their brethren. The example Jacob uses is the wicked desire of the Nephites for wives and concubines. He explains that the Lamanites are more righteous in their family relations, for their husbands love their wives, and their wives love their husbands. The history of the Book of Mormon shows that, in general, the Lamanites were faithful to their own family members. The crimes and wickedness of which they were guilty were usually directed against others. The fact that they had integrity in their family relations was acknowledged by the Lord, wherefore, because of this observance, in keeping this commandment, the Lord God will not destroy them.
This was not the only time when the Lamanites were more righteous than the Nephites. About 30 BC, Nephi and Lehi converted thousands of Lamanites. They became a righteous people—they did preach with exceedingly great power and authority (Hel 6:5) and cast the Gadianton robbers out of their land (Hel 6:37). Their righteousness culminated in the ministry of Samuel the Lamanite. In the year that Samuel came to prophecy (6 BC) it says, the Nephites did still remain in wickedness, yea, in great wickedness, while the Lamanites did observe strictly to keep the commandments of God (Hel 12:1).