Steve -- I have now looked at the link you provided for this family and see that you did have a lot of the information already.
I hadn't realised that initially, so we both have told you a lot of what you already knew.
John junior though, was born in 1824 in Halton and not 1825 -- and was baptised at Halton.
Sarah Ann Smith child of John senior and Sarah was also baptised at Halton in 1823 (she was buried there it seems on 1st August 1824 aged 18 months. (Another Sarah Ann was born on 28 Sep 1826 at Weston Turville - - buried 14 March 1838 at Weston Turville.
However I think you know all that already - and I am just correcting the actual birth year you are showing for John junior.
So James and his wife, Charlotte Fryer married Q/E March 1868 in Canterbury (Kent) Registration District. (They must have met whilst he was in the Army).
The couple had 3 children -- mother's maiden name Fryer in all cases - AND all born in Alnwick Registration District.
William born Q/E March 1871
John born Q/E Dec 1874 --- probably died Q/E June 1876 Alnwick Reg Dist - aged 1
George Vincent born Q/E June 1877
If you are now wanting to follow William and George Vincent -- they should be on all the census records from 1881 on both Ancestry - which your brother has access to - or the census records on Family Search.
In addition you should be able to find any marriage - and any children born to them. Just in case you don't know - once you know a wife's maiden name from a marriage record - you can find births of children to that couple, by using just the father's surname - the mother's maiden name - and searching for 5 years at a time - male and female separately -- on the GRO Index. (GRO is General Register Office).
It's a free site, but you need to register first;
https://www.gro.gov.uk/gro/content/certificates/login.aspthen go to; Search the GRO on line index.
In case you don't know George Vincent Smith is included on the Probate Register on Ancestry -- this states he died 4 January 1964 with effects of £2,349.00 with probate granted to Thomas Henry Defty Horne, Bank Manager.