Jenny, It looks like they are missing from the census, at least from the transcription. Suggest as a last resort you browse through the census for the area to see if they are there and for some reason have been missed by the transcribers.
This is the best I could find, but the age of Mary seems wrong.
1871 census - Birchy Field Lane, Oldbury, West Bromwich, Staffs.
James Brazier, 53yrs, Labourer (colliery),(birth column empty).
Mary Brazier, 48yrs, born Tipton, Staffs.
Sarah Brazier, 13yrs, born Tipton, Staffs.