Hi Julie ...
I presume that you're actually looking for Agnes and Winifred on the 1891 (not 1991!) census.
Have found Agnes quite easily - she's a servant in the household of Ellen H(e)illman, Widow, at 51 Lime Hill Road, Tunbridge, Kent.
Unfortunately, have not found any obvious entry for Winifred. She may, of course, be married. There is also a death on FreeBMD for a Winifred HART in 1886 (aged 13) in Bolton.