I presume that Mary Ann TOWERTON is the one who appears in the GRO birth indexes, registered 1889 in Lambeth RD?
There is a possible brother - Henry Charles TOWERTON - also registered in Lambeth in 1889.
However, I can see no birth registered for your nan, Ellen. The two closest are Nellie (often a pet name for Ellen) TOWERTON (1876 in Wandsworth) and Ellen Martha TOWERTON (1880 in Kensington).
From subsequent census and marriage records, I can definitely confirm that neither of these is your nan (Ellen Martha is the daughter of John and Charlotte TOWERTON, and Nellie married in 1898 and is still married to the same chap on the 1911 census).
A couple of questions about Mary Ann's birth certificate:
1) What is father Henry's occupation?
2) What exactly is written in the mother's column - it should be something along the lines of "Ellen Eliza TOWERTON, formerly (maiden name)"?