Complex sentence: The Nizari Ismailis recognize the Imamat of the descendants of Ismail ibn Jafar, eldest son of Imam Ja'far al-Sadiq, while the larger Twelver branch of Shi'ism recognize the Imamat of the descendants of Ismail's younger brother Musa al-Kazim.
Split into simpler sentences: The Nizari Ismailis recognize the Imamat of the descendants of Ismail ibn Jafar, eldest son of Imam Ja'far al-Sadiq. The main branch of Shi'ism, on the other hand, Twelver, recognizes the Imamat of the descendants of Ismail's younger brother Musa al-Kazim.