"And inasmuch as they bretheren shall rebel against thee, they shall be cut off from the presence of the Lord."
Because Nephi set the example his brothers had something to follow for the blessings they could have gotten. But as Laman and Lemuel were both so hardened they would lose their ability to be in the presence of God ultimately. No unclean thing can be in the presence of God. It's by keeping the commandments of God we are able to come unto Him again.
Nephi, himself, establish example. His brothers see and follow can, but choose not. Those-two able to approach God not. Thing itself dirty with God can't. Obey Heaven Father commandment with him again can.