Ishmael is a descendant of Ephraim (see Journal of Discourses, 23:184–185). He and his family live in Jerusalem at the time of Lehi’s departure. Lehi is a descendant of Manasseh (see Alma 10:3). Thus both families are of the lineage of Joseph of Egypt through his two sons. Ishmael’s family included his wife, five daughters, and two sons, “and their families” (1 Nephi 7:6). Presumably the two sons already had wives and children. With five daughters of marriageable age, the mathematical situation within Ishmael’s family was conducive to an arrangement for marriages with the four sons of Lehi, plus Zoram, the latter receiving the oldest daughter as his wife (see 1 Nephi 16:7).
Ishmael does not live to complete the journey to the promised land. He dies in the wilderness “and [is] buried in the place called Nahom” (1 Nephi 16:34), but his family continues on to contribute to the founding of mighty nations.