Based on your additional information that she married William Stanley Prince, the following information from the 1939 register should help:
Name: Mary E Prince
Gender: Female
Marital status: Married
Birth Date: 12 Jan 1890
Residence Year: 1939
Address: 2, Mount Pleasant
Residence Place: Bideford, Devon, England
Occupation: Unpaid Domestic Duties
Schedule Number: 2
Sub Schedule Number: 2
Enumeration District: WHEF
Registration district: 286 (1)
Inferred Spouse: William S Prince
Household Members:
Name
William S Prince
Mary E Prince
Ernest Beer
Source: 1939 England and Wales Register The National Archives; Kew, London, England; 1939 Register; Reference: RG 101/6858E https://search.ancestry.co.uk/cgi-bin/sse.dll?db=1939UKRegister&indiv=try&h=29719423
Image (should be visible even without an Ancestry subscription): https://www.ancestry.co.uk/sharing/16003954?h=913629
The birth date for William S. Prince is given as 23 July 1889 and for Ernest Beer is 13 December 1891. I think Mary Ellen and Ernest mis-stated their years of birth by 1 year and they should actually be January 1889 and December 1890, as I believe these are the birth registrations for her and her brother Ernest, both with mother's maiden name Littlejohns:
BEER, MARY ELLEN LITTLEJOHNS
GRO Reference: 1889 M Quarter in BIDEFORD Volume 05B Page 514
BEER, ERNEST LITTLEJOHNS
GRO Reference: 1891 M Quarter in BIDEFORD Volume 05B Page 509
Source: GRO index search (https://www.gro.gov.uk/gro/content/certificates/indexes_search.asp)
Other possible siblings:
BEER, JAMES LITTLEJOHNS
GRO Reference: 1882 J Quarter in BIDEFORD Volume 05B Page 533
BEER, JOHN THOMAS LITTLEJOHNS
GRO Reference: 1883 J Quarter in BIDEFORD Volume 05B Page 510
BEER, ARTHUR WILLIAM LITTLEJOHNS
GRO Reference: 1885 J Quarter in BIDEFORD Volume 05B Page 509
BEER, FREDERICK LITTLEJOHNS
GRO Reference: 1887 J Quarter in BIDEFORD Volume 05B Page 512
BEER, CECIL LITTLEJOHNS
GRO Reference: 1893 M Quarter in BIDEFORD Volume 05B Page 499
BEER, FRANCES MAY LITTLEJOHNS
GRO Reference: 1894 J Quarter in BIDEFORD Volume 05B Page 471
BEER, LEONARD LITTLEJOHNS
GRO Reference: 1896 S Quarter in BIDEFORD Volume 05B Page 476
BEER, ALFRED LITTLEJOHNS
GRO Reference: 1900 M Quarter in BIDEFORD Volume 05B Page 471
BEER, AUBERY EDWARD LITTLEJOHNS
GRO Reference: 1901 M Quarter in BIDEFORD Volume 05B Page 440
BEER, CHARLES HENRY LITTLEJOHNS
GRO Reference: 1902 J Quarter in BIDEFORD Volume 05B Page 459