What resources are available pre-1851 census and pre-1827 civil records?

I have two IoM families, GALE and FELL. They are both found in multiple censuses on Cumberland Street in Liverpool (1851-1881) noting that they were born in Douglas, IoM.

Thomas FELL born about 1802
His wife, Isabella GALE born about 1804

Elizabeth FELL born about 1807 (later married Richard Thomas)

I suspect that Thomas FELL and Elizabeth FELL may be siblings, but do not know how to prove it. Their marriage records in Liverpool(1826 and 1829 respectively) only list marriage date, names of bride and groom and witnesses.

On the IGI, I have found christening records for children of a James FELL and Jane CARRON that somewhat match: Thomas FELL christened 10 Dec 1802 (good match), and Elizabeth FELL christened 01 Jan 1811 (seems too late) at St. Mathews, Douglas, IoM.

How does one determine parentage before civil records? Thanks for any/all advice!

-Susan