Hmmm.
We have Hugh Smith and Isabella Smith together in the 1841 census. He's 22, a butcher journeyman, and she's 28.
Then we have Hugh Smith, butcher, in 1851 with wife Mary; daughters Agnes, 6; Phyllis, 3; Janet,1; and Eliza Grierson, sister-in-law, 21. All born Troqueer except Eliza.
In 1861 there are Hugh, butcher; wife Mary; daughter Phillies, 13 and son James, 8, all born in Troqueer.
There is a death of a Janet Smith, mother's surname Grierson, in Dumfries in 1859. I would want to look at that death certificate to see if she is the daughter of Hugh Smith and Mary Grierson. If she is, I think it would tie them all together, and that would provide evidence to suggest that Isabella Smith and Hugh Smith were sister and brother.
There are baptisms of Agnes on 23 January 1844, Phillias on 1 December 1847 and James on 22 August 1852 to Hugh Smith in Troqueer, but only James' gives his mother's name, only just as Mary.
Inconveniently, there is also a baptism of a Thomas Smith, parents Hugh Smith and Mary Grierson, on 26 January 1844 in Dumfries - three days after the baptism of Agnes. So I'd want to take a look at all those baptisms to see what they say.
I also wonder what became of Hugh and Mary. I see that a Mary Smith or Grierson died in Dumfries in 1889 aged 67, which is within the range of dates of birth from the censuses.
There's a marriage of a Phyllis Smith in Dumfries in 1875. Was this Hugh's daughter?