The Middle East of the era has never been of great interest to me. So, I haven't really studied it, and the history of the foundational period of Islam is likely a mythology. If we have no contemporary history of Mohammad, when does any credible history of the religion begin?
My hypothesis is that Ali may have been a real person who died in the Abbasids' rebellions against the Umayyads c. 750 A. D., and he was then declared to be the relative of the newly invented prophet who died in a holy war. The Abbasid rebellion began in the region that is now Iran, and that is the base of power for the Shia, so that fits. If Mohammad had any blood relatives, I would expect that they would have been in Arabia, not Iran, claiming that they were Mohammad's rightful heirs and the true keepers of his faith. Yet, only in Yemen in modern times are the Shia strong on the Arabian peninsula. The population may have converted to Shia there at a much later date by order of the local chieftain.
The early Muzzie history is all BS. There are also the Abadi Muzzies in Oman who are neither Shia nor Sunni, but that is lesser known or noted.