As Vance Mead says, wills are one of the best avenues to try and find relationships.
Do you know their occupations? Did they follow in their fathers' footsteps?
Were they farmers, 'yeomen' etc? If so you may be able to find them in Rent Rolls or Manor Court records. They may have passed on leases to their children.
You would need to find which Manor your ancestor lived in, and where the records for it may be held.
Who were the local landowners?
There is a Manorial Court register on the National Archives site.
https://discovery.nationalarchives.gov.uk/manor-searchThese records are highly unlikely to be on-line anywhere,
There is still a great deal of information in depositories all over the country, that is, and will probably never be, digitised.
Have you checked The National Archives Discovery site for any mention of them?https://discovery.nationalarchives.gov.uk
And the Yorkshire record office catalogue
https://www.northyorks.gov.uk/leisure-tourism-and-culture/county-record-officeIt's difficult to just rely on parish registers at this time, as they can be incomplete.
And as Ashtone says, always check for any burials of children you find born, to eliminate them from your research.