Josiah was mentioned in the will dated 5 Feb 1718/19 of his uncle Joseph Arnold. He is identified as a son of William (of Israel & Mary [Barker] Arnold).
BSR says Josiah's parents were William & Christine Arnold. A genealogy at Rootsweb's Worldconnect has his parents as William Arnold & Hannah Nichols. Since there were several Josiah Arnolds, it is confusing, and Maplet's husband may be one of the others of the same name.