We know from carlier sources in our present Book of Mormon that Lehi was a descendant of Joseph of old (1 Nephi 5:14; 1 Nephi 6:2; 2 Nephi 3:4), but it is from the 116 manuscript pages that became lost, that had been translated from Mormon’s abridgment of the book of Lehi, that we learn specifics.
Elder Erastus Snow explained in 1882:
“The Prophet Joseph informed us that the record of Lehi was contained on the 116 pages that were first translated and subsequently stolen, and of which an abridgment is given us in the first Book of Nephi, which is the record of Nephi individually, he himself being of the lineage of Manasseh; but that Ishmael was of the lineage of Ephraim, and that his sons married into Lehi’s family, and Lehi’s sons married Ishmael’s daughters”
(JD 23:184, italics added)