Hi Justin, there is no relation. She is not the sister of the opera singer, they were the daughters of Samuel Dolby 1795–1834 and Charlotte nee Niven 1795–1866
Sarah Amy Niven Dolby
b. 27 Jun 1832 London
d. Nov 1905 Hampstead, London
who married
John Foster
b. 1828 Staines, Middlesex
d. 25 Mar 1915 Hampstead, London
Charlotte Helen Dolby who married Prosper Philippe Catherine Sainton
b. 17 May 1821 Westminster, London, England
d. 18 Febr 1885 London, England