Also another daughter
SAUNDERSON, ELVINA  WINIFRED     mmn EWANSON  
GRO Reference: 1901  D Quarter in WEST DERBY  Volume 08B  Page 610
Can be found at Wavertree in 1911 as niece to Plaiston family. Jane Ewanson also with family.
1911 https://www.ancestry.co.uk/discoveryui-content/view/23663388%3A2352
RC baptism (1 Balm Street)
Helaina Winefred Saunderson
Age   0
Birth Date   24 Nov 1901
Baptism Date   11 Dec 1901
Baptism Place   Kensington, Christ Church, Lancashire, England
Father   George Saunderson, lab
Mother   Elizabeth Saunderson
Added: George a Soldier on her marriage in 1927 to Ernest Jump. Image online.
https://www.ancestry.co.uk/discoveryui-content/view/93123961%3A2197
Cas
Hi
I am going to throw this in the mix.
It seems the Mother in Law has gained 10 years by 1911?
1911
Louisa Plaistow, Wife, married, 35, born Liverpool. 
 ... 
Jane Ewanson, Mother-in-Law, 68, Widow, Lerwick, Shetland Isle. 
Elvina Winifred Saunderson, Neice, 9, Lancs, Liverpool.
There is a Ewanson family (Snr) look to be born in the Shetland Islands and there is a child called Elvina, too.
1881
Arthur Ewanson, Head, Mar, 45, Shetland Islands. 
Jane Ewanson, Wife, Mar, 29, Shetland Islands. 
Elvina Ewanson, 7, Liverpool. 
Louisa Ewanson, 4, Liverpool. 
Christiana Ewanson, 2, Liverpool. 
My initial thought might be wrong and you may have considered Shetland or Scotland connection already?
Perhaps Elizabeth Ewanson / Saunderson was visiting in Scotland when the Census was taken? Not in E & W.
Added: 
1891 Army Record - Perhaps the N of K Elisabeth and Brother William, were on a Scottish jolly when the Census was taken?