Hello Dean
It's a pity that there are no siblings as witnesses but all is not lost.The next step would be to see if William was a witness at any of the others marriages,also wills for the parents & monumental inscriptions may give some info. but all this has to be done at the War.CRO. where the records are kept.Also does William have any of the above names for his own children.
As the info. for this period is sparse you are unlikely to find cast iron proof of a connection but there doesn't appear to be another William Dewis born at this time.
Regards Jim