On anthroponymic grounds Alpin may have been the brother of his predecessor Ciniod son of Uuredach as both have similar patronyms, the Pictish equivalent of the Old Irish Feredach, but no certain evidence exists for this identification, nor for an assumption that he was a member of the Cenél Loairn which might follow from it.
He appears to have reigned from the death of Ciniod until his own death in 780. This is misreported in the Annals of Ulster as that of Alpin, king of the Saxons, but no Saxon king named Ælfwine is known from this time. The Annals of Clonmacnoise name him Alpin, king of the Picts, and this reading is taken as the correct one.
References
Sources
Anderson, Alan Orr; Early Sources of Scottish History A.D. 500–1286, volume 1. Reprinted with corrections, Paul Watkins, Stamford, 1990. ISBN1-871615-03-8