I think that I've cracked it; here she is!
"Statutory Marriage 168/02 0099 When and Where Married: 25 August 1856 at 58 Spital Old Aberdeen Name and Profession: John Ross (Bachelor) Corporal Royal Artillery Age: 29 Usual Residence: 58 Spital Old Aberdeen Name Surname and Profession of Father: Alexander Ross Labourer Name and Maiden Surname of Mother: Jane Ross M.S. Swanson Name and Profession: Mary Greive (Spinster) Domestic Servant Age: 19 Usual Residence: 33 School Lane Woodside Aberdeen Name Surname and Profession of Father: Alexander Greive Greive at Cullen House Name and Maiden Surname of Mother: Margaret Greive M.S. Skinner"
More like sister-in-law and hardly unmarried - unless she's a widow?
She's also registered using her maiden name of SKINNER rather than her married surname of GREIVE.
Unless Mary was illegitimate, and wasn't quite truthful about her parents' marital status. This would tie in with the 1841 census, where Mary Grive, aged 3, and Margaret Skinner were in the household of Alexander Skinner in New Buckie.
There's a bit of a shortage of Alexander Grieves, however. The only one in Banffshire in 1851 is a widower, aged 31, farm labourer at Brountoun in the parish of Rathven. He is probably the one recorded aged 24 in the 1841 census, a farm servant in Buckie.
Interestingly, the one in 1841 is in the household of James Kessack, and there is a marriage of an Alexander Grieve to Isabell Kissach in Rathven in 1839. LIBINDX
http://libindx.moray.gov.uk/people/mainfilter.asp has a reference to this marriage from the Roman Catholic register, which says that Alexander's parents were James Grieve and Helen Murdoch, and that Isabel's were James Kessoch and Helen Murdoch. (Yes, both mothers named as Helen Murdoch.)
They had three children: James 1840; Isabella 1841; Alexander 1843. I have failed to find any of the three children in the 1851 census so far.
In 1853 one Alexander Grieve married Helen Geddes in Enzie, and this couple are in Enzie, he described as a salmon fisher, aged 47 in 1861 and in Rathven aged 70 in 1881. So again curiosity got the better of me.
Alexander Grieve, general labourer, aged 75, married first to Margaret [sic] Kessack and second to Helen Geddes, died in Buckie on 25 March 1888. His parents were James Grieve and Helen Murdoch.
So this is what I think the story is.
Alexander Grieve is born in 1811 to James Grieve and Helen Murdoch.
In 1837/8 he fathers an illegitimate daughter, Mary, by Margaret Skinner
In 1839 he marries Isabella Kissack (many spelling variations possible)
In 1840 their son James in born
In 1841 Alexander is recorded in the household of his parents-in-law, described as a farm servant, aged 24
In 1841 their daughter Isabella is born
In 1843 their son Alexander is born
Between 1843 and 1851 Isabella Kissack of Grieve dies
In 1851 Alexander is recorded, a widower aged 31, farm labourer, in Brountoun
In 1853 Alexander marries Helen Geddes
In 1856 Mary marries, and describes her father as a farm grieve at Cullen House
In 1861 Alexander is described as a salmon fisher and living in Enzie, aged 47
In 1881 he is also described as a salmon fisher and living in Enzie, aged 70
In 1888 Alexander dies aged 75, described as a general labourer
You can, and you should, verify all of this by looking up the original documents on SP.
None of which gets anyone any further with the parentage of Alexander Ross
