If it is the correct Ada on the 1911 census, this her and her sister’s birth:
ALLEN, ADA mmn PERFECT
GRO Reference: 1897 M Quarter in AMERSHAM Volume 03A Page 711
ALLEN, CLARA JANE mmn PERFECT
GRO Reference: 1899 J Quarter in AMERSHAM Volume 03A Page 751
Marriage:
Mar 1888 Amersham 3a 579
ALLEN George
PERFECT Ellen
Jamjar