It seems there are several Welborne families, and they are *not* related.
Captain Thomas Welbourne is the founder of the Accomack/Northampton County line, and not all of his descendants are known. His parentage is somewhat uncertain, but he probably belonged to the Welbourne family of Oswaldkirk, Yorkshire. There was a William Willbourne in Accomack before him (636), probably a relative, possibly his father.
Edward Welborn, presumptive founder of the Maryland line, came from Lincolnshire and was *not* the son of Thomas Welburne of Accomack. There were Welbornes in Maryland before him, a Samuel in 1659 and a Robert in 1662, but where they came from, and whether they were related (to each other, to Edward, or to anybody) is unknown.
Both Thomas' and Edward's lines prospered and proliferated, and occasionally seem to have been found in the same territory (the Carolinas, Tennessee, etc.). As a result they get thoroughly tangled up with each other and with other Welborn/Welburn/Wilburn families, and sorting them out is incredibly difficult.