In 1 Nephi 1:28 Lehi gave a conditional blessing to Laman, Lemuel, the sons of Ishmael, and Sam. He did not give them a blessing at that time. It appears that they had rights upon the birthright blessing, and that was their blessing, although Lehi certainly saw that the conditions would confer it upon Nephi. This would leave those sons’ posterity without a blessing. In this chapter, Lehi provides the posterity of those men a blessing in lieu of the one that would have been given to their fathers.
It may read too much into Lehi’s statement, but note how he addresses Laman’s posterity: “Behold, my sons, and my daughters, who are the sons and the daughters of my first-born” (emphasis mine). Seen in the light of the missing blessing on their father, it may be that Lehi is suggesting that Laman (and later Lemuel where he uses similar language, v. 9) have lost their place in the family and that Lehi has ritually elevated their children into his own family to replace them. Hence they are technically his grandchildren, but from the standpoint of a patriarchal blessing, his children.