I have not researched this group in a while, but I have that Priscilla was the daughter of Allison Mosby who died abt 22 Oct 1763, shortly after her birth in 1762. And that Allison Mosby was the son of Jacob Mosby and Susannah Cox. Jacob and Susannah both died abt 1781. It would appears that Priscilla's brother,Jacob Mosby, was named after his grandfather. The Will of Jacob Mosby might shed some light on Priscilla.
Some researchers give Allison's name as Stephen Allison Mosby. I have not researched this point.