ScotlandsPeople has always been good given that you had immediate access to certificates for a reasonable price - if you weren't searching a name like Smith or Jones!!! A major improvement to be able to search for free.
When you say you have copies of the census from 1851 & 61 do you mean the actual images or an Index? I would suggest you need the images to confirm exactly what relationship Robina is to Charles Chapman (or at least what the image shows). The indexes I've seen list her as a daughter in 1851 and niece in 1861.
If she is his niece then it raises the possibility that both John Chapman 's are related (cousins) - the father of the second being a brother of Charles.
Just to add to the complication the death notice you attached lists her father as George who appears to have been in the book industry.