Appears as if John did indeed have a sister named Martha Ann -

Births December Quarter 1881
BRADLEY Martha Ann Northallerton Volume 9d Page 618

1901 Census -

John Wm Bradley Head England and Wales Census, 1901 1874 Northallerton, Yorkshire Brandon And Byshottles, Durham, England
Martha Ann Bradley Sister England and Wales Census, 1901 1882 Northallerton, Yorkshire Brandon And Byshottles, Durham, England
Muriel Bradley Niece England and Wales Census, 1901 1900 Northallerton, Yorkshire Brandon And Byshottles, Durham, England

1911 Census -

BRADLEY LANCELD M 1910 1 Chester le Street Durham
BRADLEY HARODL JAMES M 1907 4 Chester le Street Durham
BRADLEY JOHN WILLIAM M 1874 37 Chester le Street Durham
BRADLEY JOHN WILLIAM M 1902 9 Chester le Street Durham
BRADLEY MARTHA F 1882 29 Chester le Street Durham
BRADLEY OLIVE MAY F 1906 5 Chester le Street Durham