Hi Rosie,
Did you sort this out?
I note the following on FreeBMD:
A Frederick James TEBBS was born Sep quarter 1844 (which fits with the age at death of the Frederick James TEBBS mentioned above)
A Frederick James TEBB (sic) married Sep quarter 1872, Lambeth, Volume 1d, page 618; there is a Mary Ann HARRIS on the same page.
So it seems that the couple may very well not actually have been married at the time of the 1871 census.
A Mary Ann TEBBS age 28, and a Mary Ann TEBBS age 0, died St Saviour, Dec qtr 1872. I wonder if this is your Mary?
In the 1881 census on FamilySearch, there is a Sarah TEBB (sic), age 11 born Lambeth, a Scholar - an inmate at the Lambeth Workhouse.
This could very well be your Sarah.
If both Frederick and Mary were dead, this would explain Sarah's presence in the Workhouse. And perhaps her HARRIS grandparents had also passed away by then.
JAP