Hi Jane,
I think this could be your Helen Sinclair on 1851 census.
Burn of Halkirk Macdonald William Head U M 41 Farmer of 8 Acres b Halkirk
Macdonald Hellen Widow F 80 Farmer's mother b Halkirk
Macdonald Iso. sister U F 34 Working on farm b Halkirk
1841 Census Sibster Halkirk
Macdonald Willm. M 30 Labourer b Caithness
Macdonald Helen F 60 b Caithness
Isabella F 20 b Caithness
from IGI children of George McDonald & Helen Sinclair all baptised Halkirk
29 Oct 1800 Christian
28 Jul 1803 George
7 Jan 1806 William
7 Feb 1809 Isobel
30 Jun 1810 John
13 Jul 1815 Isobel
1851 Census Burn of Halkirk
Macdonald George Head M 47 Farmer of 7 Acres b Halkirk Caithness
Macdonald Williamina Wife F 30 b Sutherland
Macdonald Mary Dau F 5 b Halkirk
Macdonald Christy Dau F 3 b Halkirk
Dau F 14w infant unbaptises b Halkirk
1861 Census Halkirk
Macdonald George Head M 50 Carter b Halkirk
Macdonald Williamina Wife F 35 b Sutherland Strathy
Macdonald Christina Dau F 10 Halkirk
Macdonald Johannah Dau F 8 Halkirk
Macdonald George Son M 6 Halkirk
Macdonald Barbara Dau F 2 Houstry
I got them on Freecen
http://freecen.rootsweb.com/cgi/search.plKris