Hi Fordyce,
From Freereg:
James Sibbald, b. 4th August 1817 (bapt. 8th September) - witnesses William Farguson [sic] and Adam Elder
John, b. 12th May 1820 (bapt. 11th June) - Arch. Watson and William Fergie
Marion, b, 18th December 1822 (bapt. 25th December) - William Fergie and Andrew Cathie
Thomas Scott, b. 25h August 1825 (bapt. 2nd September) - Mr. Aitheson and William Irving
All in Haddington to John Preston and Marion/May Watson ...
1841 census:
Address: Church Street, Haddington, Haddingtonshire
John Preston, m, 70, slater (journeyman), b. England
Marion Preston, f, 60, -, b. in county
James Preston, m, 20, blacksmith (journeyman), b. in county
Thomas Preston, m, 15, blacksmith (apprentice), b. in county
I assume you have found the bapt. of a Marion Watson in Haddington, 1782 to Adam Watson/Marion Tutch? This would fit well with the above.
John's birth/parentage will be difficult I'm afraid. His marriage will likely not include any details because it's pre-1855 so you may need to rely on naming patterns.
Cousin wasn't always used in the same way we use it today, same with "step-[relation]". The connection could be further out than first appears.
hume