Jolyn,
Looking at the 1871 RG10, 5372, 31, 12 /1881 RG11, 5297, 4, 4 census, if Mary's sister is Emma (on 1881) and her father is George occ. Farmer (1871) then her mother is also Emma - please correct me if I am wrong.
Based on this I have found the following in FreeBMD
George Ball / Emma Watts Q3 1853 Ref: 5c 968 in Frome (Somerset)
George JAMES Ball / Emma Lewis Q3 1854 Ref: 1c 40 (East London)!!!
I cannot find a birth ref. at the moment, though.
Viv