There is an 1891 entry for an Edward and Sarah Higgs both 19 - unmarried brother and sister.
A one year old Ellen Higgs b St Pancras is shown as niece to Edward who is head of household.
Sarah could be her mother which means Ellen was illegitimate
and a possibility for Edward & Sarah Higgs bn St Pancras on 1881 census with widowed father & siblings
11 & 12, 11, Fortess Mews, St Pancras, London & Middlesex, England
Edward Higgs Head Widower Labourer Tring, Hertfordshire
Harry Higgs Son Labourer Tring, Hertfordshire
William Higgs Son St Pancras, Middlesex
Isabella Higgs Daughter St Pancras, Middlesex
Edward Higgs Son St Pancras, MiddlesexSarah Higgs Daughter St Pancras, Middlesex Mary Higgs Daughter St Pancras, Middlesex
Piece 224 Folio 19 Page 32
1871 census shows Edward Higgs born Hertfordshire is married to Isabella age 32
Piece number 244 Folio 70 Page 53
As Carole mentioned you need to find the correct birth entry for Ellen.
You could check to see if you can find a baptism record for the Ellen Higgs born St Pancras from 1891 census to see if her mother is named Sarah Higgs
Don't know if this is the same Ellen as per 1891 census
Ellen HIGGS bn 1890 Clerkenwell London Niece Head of household is Elizabeth MURRAY
1911c
https://www.familysearch.org/ark:/61903/1:1:XWGJ-5B5