Hi
I see on the 1881 census, Samuel is shown as unmarried and he and Eliza are living with his parents. Unless its on her birth certificate you may never know. Possibly she was the result of a "one night stand" and her mother didn't want (or couldn't) bring her up.
Eliza Florence V Brandon born 4th quarter 1872 District Aylesbury. Volume 3a page 540.
Looks like you'll have to buy the birth certificate.
Joan