Siegfried Frank was a son of Izaak Frank and Fanny Reingenheim. His mother lived with him.
On 5 July 1922 he married Sophie Josephine Sachs. They had two children, Julius (1923) and Emma (1926) .
An 24 February 1929 his wife Sophie died.
Siegfried Frank remarried Sophie's older sister Nelli Serline Sachs on 9 September 1931.
In this second marriage two other children were born, Fanny (1936) and Louise Bertha (1938).