Her parents were John Petrie (soldier) and Elizabeth Bain.
The marriage I posted above only took place in 1847 but you say Elizabeth Petrie was born in 1815 but you don't say where?
What census info do you already have?
John Nairn was born in Cupar in 1807.
The only 1851 entry I can find for a John Nairn b Cupar shows his birthyear as 1813 and his wife Elizabeth as born 1811 Ceres
Can you please confirm if this is the correct 1851 entry?