Re my previous posting, I've been having another look, and I am somewhat confused now. So a bit of thinking aloud here! Apologies if it doesn't make sense!
I have the Census form you mentioned, and I looked at it again, and agree that her age appears to be 45, giving a year of birth of c1796.
On my Genes Reunited tree, for some reason, I had her year of birth as 1776, which would make her 65 in 1841.
Now, I have a number of children for James Sinclair and Margaret Blackadder - as per the following from Scotland's People:
No Date Surname Forename Parent Names/Frame No. Sex Parish City/County GROS Data Image Extract
1 13/02/1817 SINCLAIR ALEXANDER JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
2 10/08/1815 SINCLAIR BEATRIX JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
3 18/05/1811 SINCLAIR JANET JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
4 24/03/1804 SINCLAIR JEAN JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
5 16/04/1805 SINCLAIR JOHN JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
6 28/07/1813 SINCLAIR MARGARET JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
7 26/04/1807 SINCLAIR WILLIAM JAMES SINCLAIR/MARGARET BLACKADDER St Ninians /STIRLING
The youngest being born in 1804. Fits with Margaret being born c1776.
However, another relative and I had a theory. Could the Margaret Blackadder on the 1841 Census be a niece rather than a wife? Given the birth in 1804 (and others) with parents John Sinclair and Margaret Blackadder, is it possible the wife died before 1841 and her niece moved in? A niece also called Margaret? Could this explain the use of the name Blackadder rather than Sinclair in the Census?
Any other theories?
David