I cannot personally locate the parish record of the marriage online but usually if no father is listed it would indicate illegitimacy.
What does concern me here (and sorry to sound a wet blanket) is that the birth of a Sarah Elizabeth Johnson in Mar.qtr.1876 Hexham, Northumberland which you think could be her is not, I feel, the correct one. On the 1911 Canada Census month of birth is given and it states July 1877 for Sarah. George's states Sept.1878 but this should clearly be 1877 as he was baptised early 1878. Whilst Sarah's birth year could equally be slightly wrong I doubt very much that the month of birth would be wrong and a July birth wouldn't be registered in a March quarter.
I would think you do need the full details from the marriage certificate - a) to ascertain whether or not a father is listed for Sarah (and not just an error on the entry you previously viewed) and b) hopefully an occupation and address for her to enable a search to find her on the 1901 Census. Unfortunately you are dealing with a common set of names for her and can't really second guess it.
Annette