Hi Joanna, Nice to hear from you...I think you're on the right track with your searches.I have managed to get the 1871, 1881 & 1891 census for Robt.In 1871 he's with his parents, John Aitken aged 38, an Engine Fitter born Dalston, Cumberland and Emma aged 30, born Newcastle.Robt is aged 6, a scholar, born Newcastle but unfortunately there aren't any siblings.
I would say that the 1881 census I referred to previously with Robt aged 16, a nephew, living with Walter & Mary A Scott is correct.I did a marriage check to find that a Walter Scott married Mary Ann Aitkin Jun Qtr 1854 in Newcastle T 10b 102.She was born Brampton, Cumberland so we might assume from that that she would be John's sister?
In 1891 Robt is a boarder aged 26 in Hornsey, Middlesex born Newcastle, Northumberland.
If you would like a copy of the images please contact me on firstname.lastname@example.org and I will forward them.I have access to anc.com via our local library (Australia) and at our family history group so will see if I can trace John and Emma back a bit further next week.
Dalston, where John was born is barely a stones throw from the Scottish border so it's possible the family originated in Scotland if you were able to go back another generation or two to find out!
Scottish death records would give you the names of the parents on John and Emma's death cert but I'm not sure you'd be that fortunate with Northumberland records.Researching can be very frustrating in that respect.
Let me know if I can help any further, cheers for now,