Mary is with John’s parents, Alexander and Catharine in the 1861 census, listed as single. For this reason I think she was born a Campbell. A corresponding birth is listed in the family Bible here in the same hand as John’s. I speculate for these reasons that she is John’s sister, though she might also a cousin.
There are several other Campbells in the same list, whose births span the years 1823 to 1838. One of the things I have yet to do is follow up on this list, and see whether I can find out if they were all children of Alexander and Catharine, or what.
I’ll send you an email when I get a chance, and we can talk more. Good to know about the Ridge, and another Lothian marriage. Lots of information out there between all of us descendants; great to share!