Hi Wiggo, welcome to Rootschat
Can I ask where you have tried looking so far to prevent duplication of effort? The official source of BMDs in Scotland is
www.scotlandspeople.gov.uk so if your grandmother was born in Scotland after 1855 she should be on there. It is a pay-per-view site, but quite reasonably priced.
www.familysearch.org is another very useful website, although Scotland coverage stops around 1874, and there are gaps, although it is free, so always a good place to start.
Have you tried looking at the census records? Scotland's people have them, and there is also the FreeCen project which is gradually transcribing them.
www.freecen.org.uk/ There are other sources of census data but the transcriptions can sometimes leave a lot to be desired!
If you can find them in the 1901 census that should give you somewhere to start. Do you know if the family staying around the Lothians of if they moved away?
1901 census search on SP shows 10 matches for Matilda Cameron, and 1 for Matilda Kelso, so a good place to start.
Have you tried searching for the death certificates of David and Matilda? They may give names of other children, depending on who registered the death. It should also give an address which might help.
There is no marriage showing in Scotland before 1933 for David and Matilda. Did they show as married on Ellen's Birth certificate?
Where in Leith were they living at the time of Ellen's birth?
Sorry lots of questions an no answers yet!
Kirsty