Looks as though Janet Marsh Doxsey had a sister / relative Gladys Marsh Doxsey born in Middlesex in 1887 and who died in 1889 in Middlesex. Gladys would not have appeared on a Census unfortunately.
Am thinking that the given name of 'Marsh' is significant ...
mother’s maiden name
Helen Susannah
MARSHname on same page
Joseph
DOXSEY Dec 1885 Thame vol 3a page 1165
Helen died 20 Feb 1919
2 Cumberland Mansions Hampstead Middx
Probate London 12 May to
Albert Benjamin Doxsey bank manager
Albert Benjamin Doxsey of Durelston Heathgate Hampstead Garden died 16 Mach 1923 Probate to widow Annie
Albert is a younger brother of Joseph Doxsey (Helen's husband)
they are the sons of Isaac & Mary Doxsey
London Electoral Register show
Janet Marsh Doxsey (dates 1914 - 30) & Charles Marsh Doxsey (1919,20,21), Joseph (1925,26)
were living at that address