A Mary Buchan married a James Sutherland in Edinburgh on 25 Dec. 1853. Children showing to them seem to match those older children showing in the household with James and Martha in 1871:
Elizabeth 1855 - Edinburgh
James 1857 - Glasgow
Jessie 1859 - Glasgow
Isaac 1861 - Glasgow (High Church), likely died young.
Isaac 1863 - Glasgow (High Church)
I see from your post on another forum that the family left for Australia in 1877. Hopefully further details on potential first marriage for James is new to you and gives you another avenue to follow up on

There is a death showing on the pay to view site
www.scotlandspeople.gov.uk for a Mary Buchan/Sutherland in 1864 in High Church, Glasgow.
Given the occupation you have for James, I wonder whether this could be him in Edinburgh before his marriage to Mary Buchan in 1851:
James Sutherland 21, printer compositor b. Edinburgh
Anne Sutherland 23, sister b. Edinburgh
Elizabeth Sutherland 17, sister b. Edinburgh
Address: Pleasance, Edinburgh
From this 1851 entry, this may be James in 1841:
James Sutherland 50, shoemaker b. Scotland
Jannet Sutherland 40 b. Scotland
Isabella Sutherland 20
Alexander Sutherland 15
Ann Sutherland 14
Jane Sutherland 10
James Sutherland 9Elizabeth Sutherland 5
William Sutherland 3
Address: Potterrow, Edinburgh
All the children showing as born in Midlothian on this 1841 census entry. Names of parents, James and Janet, would fit well with the names of James Sutherland and Mary Buchan's first born children. Jessie is a common variant of Janet in Scotland.
Monica