On Familysearch I have found 10 children (all with various spellings of Hairstanes) born between 1717 and 1732.
Be careful with any information you find online, especially in the 'community contributed' part of the IGI on Family Search. Use it as a finding aid, and make a point of checking the original documents.
Also in the Edinburgh register is the marriage of her niece, Isobel, to Ebenezer McCulloch, merchant of North Kirk parish, in 1774. It says she is Mrs Isobel Hairstanes. That would mean she had been married before wouldn't it?
Absolutely not.
In Scotland a woman does not legally change her surname on marriage, which is why you get the mother's maiden surname in most Scottish baptism records (if you get a mother's name at all, that is!). Until the early 19th century it was customary for a woman to be known by her maiden name all her life. You often find wives listed in the early census by the maiden names, and references to 'Mrs Jean xxxx wife of John yyyy.'
Also a middle or upper class woman was accorded the title 'Mistress', which is what Mrs is an abbreviation of, just as nowadays in French a woman is 'Madame' and in German 'Frau', in Russian 'Gospodina', in Italian 'Signora', in Spanish 'Seņora' and so on irrespective of her marital status. English is the only major European language that demands to know whether or not a woman is married before deciding how to address her. So in 1774 'Mistress Isobel Hairstanes' means that Hairstanes is her maiden name.
I have found in 18th century registers references to the baptisms of children to 'Mr Alexander Leslie and Mrs Anne Duff his spouse' and similar.
There's also the famous song by Lady Nairne, when the Laird o' Cockpen goes to ask his neighbour's daughter to marry him, and the young and obviously unmarried lady is consistently referred to as 'Mistress Jean'.
