So if we think that Hugh and Isabella in the 1841 census are brother and sister, if would follow that Isabella's parents were James Smith and Mary Reid.
What does Hugh's death certificate say was the occupation of his father James Smith?
Unless there were two James Smiths both married to Mary Reids, James and Mary got about a bit. See screenshot.
In 1851 James Smith, shoemaker, born Mauchline, is in Glasgow with wife Mary, 58, born New Cumnock and daughter Margaret, 21, born Mauchline, and a grandchild.
They might be the family in Fenwick in 1841: James, shoe maker, 50; Mary, 45; Mary, 19; John, 14; Adam, 16; William, 12; Margaret, 10; all born Ayrshire.
But if Isabella was 28 in the 1841 census, she was born in 1812/13 and Hugh was born 1818/19. Isabella's YoB is not a problem, but for Hugh to the theirs they must have moved briefly from Ayrshire to Troqueer, and then Hugh and Isabella moved back to Troqueer, so this family is probably a red herring.