OK, not knowing Mary's age I took a chance and purchased the registry entry for the birth in Glasgow and it looks like we may have got lucky.
Mary Cameron McGhee born 15 Dec 1889 at 181 Stirling Road, Glasgow*
Birth registered by father on 3 Jan 1890.
Father George McGhee, railway companys parcel deliverer
Mother Catherine m.s. Campbell
They married 24 June 1887
Someone can maybe look up 1901 census for you which will give names of siblings in 1901 and address where living then (and father's occupation).
I'm getting this info from ScotlandsPeople
(
http://www.scotlandspeople.gov.uk )
This is a pay per view site. You'll need to register but costs are very reasonable. I've only spent a couple of pounds to find out this much. Minimum purchase is £6GBP for 30 credits. A copy document costs 5 credits.
It should be easy for you to locate the marriage of George and Catherine given that you now have their names and date of marriage. That in turn will give you their agesand both sets of Mary's grandparents.
This all looks very hopeful but you need to try and find Hanna's birth to confirm we have correct family.
I'll have a quick look for some info on Donald.
Dave
*PS, my grandmother worked in a bakery in Stirling Road. I can send you a photo if you want.
I'll send you a PM with my email address. That way I can send you the birth record I found. No point in you using credits for it as well.