Hi Gina,
Welcome to Talking Scot!
Betty Turnbull appears in Lairg in the 1841 census, but unfortunately is not with her parents. She is only 20, but is the oldest in the household. Makes one wonder who the children were!
1841
Forbole, Lairg, Sutherland
Bettrie Turnbull, 20, born Sutherland, Servant W Widdow Mcdonald Abrent
Isabell Ross, 13
William Loggan, 4
(sorry, the ancestry.com transcription is a bit odd!)
It's not proof, but there only seems to be one Turnbull family in Lairg in that time frame. Some of the births are on the IGI, as I'm sure you have seen. They appear in 1841:
1841
Rean, Lairg, Sutherland
William Turnbull, 50, born England, Shepherd
Jean Turnbull, 45, born England
Magdalene Turnbull, 18, born Sutherland
Ann Turnbull, 15, born Sutherland
Jean Turnbull, 15, born Sutherland
Jemina Turnbull, 8, born Sutherland
William Turnbull 7, born Sutherland
Helen Turnbull, 5, born Sutherland
Both the parents were born in England, so perhaps they were married there also. In 1851 they conveniently say they were both born in Northumberland, narrowing the search down considerably!
Was John and Betsy's first child Margaret? If so, they might have been still in Scotland for the 1851 census and their birthplaces will be listed:
1851
Torroble, Lairg, Sutherland
John Ross, 33, Head, born Kincardine, Rosshire, Labourer
Betsy Ross, 32, Wife, born Lairg, Sutherland
Margaret Ross, 6 Mo, born Lairg, Sutherland
All the best,
Sarah
Later: The Turnbulls have all disappeared from Lairg by 1861. Did they all go to Australia?