My thoughts exactly Toni

Looking in the Windsor area in the 1881 census, there's an Elizabeth b1877, with her mother Elizabeth and presumably sister ALICE (6 months old) - this Alice has no Birth Index entry that I can see ... and she is younger than your facts suggest ....
RG11/1325; Folio: 107; Page: 3 - Windsor Workhouse ...Elizabeth Attwell 24 Inmate Servant New Windsor
Elizabeth Attwell 4 dau Windsor Workhouse
Alice Attwell 6 months dau do
Might be worth seeing if this Elizabeth married, and thus what name the young Alice has in 1891 (if she survived)
Also a backtrack on Elizabeth to establish her parentyage ....