Hi Colin
Just because we all like the fact and records to be precise
...
I think the marriage of William and Agnes Snr was not in Greenock but in Govan from what we had on the index?
There is this marriage showing in 1872 in Govan https://familysearch.org/ark:/61903/1:1:XTBJ-VYS
Not sure if anyone your side researching has looked at these original records which are available to view online on
www.scotlandspeople.gov.ukAlso, we haven't seen any reference as yet to another sister for Agnes (may have been born and died between the censuses) but there certainly seems to have been another son, Francis, who shows in 1881 as 7 months old, when the family were living at 2 Camden Place Govan Road, Govan (census entry above). Likely he died early too as he doesn't show in later censuses.
Have a look at this info on the only ever Mayoress of Jo'burgh
http://joburg.org.za/index.php?option=com_content&task=view&id=241&Itemid=51&limitstart=1 This was not until the 1940s. It looks like it was Agnes' husband John Christie who was Mayor in 1920-1, although naturally with wife Agnes part of that life for that year.
Interesting further snippets here, from google books (pg 126 onwards)
www.rootschat.com/links/01e2r/ - shrunk link and here
http://en.wikipedia.org/wiki/John_Christie_%28mayor%29Monica