When you say "it cannot be documented that John and Thomas were sons of Arthur," I presume you mean you have not personally found documentation. Good news is, they can in fact be documented, and are, as follows:
1737: Arthur Onby deceased in Essex Co., Virginia;inventory of estate:
The inventory lists 3 surviving children: JOHN Ownbey , Mary (who married James Williamson), and Elizabeth (who married Carter Wells).
The inventory also lists his grandson Thomas, "the son of THOMAS Ownby and Mary Brooks Stamper."
The foregoing proves that John was his son, and that the late Thomas was his son.