I had a look on
https://www.freecen.org.uk/cgi/search.pl to see if there were any Stewarts there.
In 1841 the only one was Cecilia Stewart, aged 24, whom I take to be your one.
In 1851 there is a John Stewart, ploughman, 36, born Culter, with wife Rachel and two children. Is this your John?
I rather fear that their parents must have died before the 1841 census. If so, then there may not be any record of their deaths, because there was no requirement to record deaths until the start of statutory civil registration in 1855.
There are three avenues of enquiry that you could try.
First,
www.scotlandspeople.gov.uk has deaths and burials from the parish registers.
Next, the kirk session records of Wiston and Roberton (there's no 's' in Roberton) may include mortcloth records. These are held in Edinburgh and can be accessed in digital form in the Historical Search room of the National Records of Scotland, or in some local archives including those of Glasgow, Hawick, Aberdeen and Inverness. The mortcloth was a cloth that was draped over the coffin at a funeral. Most parishes had at least one mortcloth which they rented for funerals, and the parish accounts often say who was being buried when the fee for renting it was recorded in the cash books.
Third, there might be a gravestone, and if so there might be a transcription of it in one of the booklets published by the Scottish Genealogy Society.