There is no doubt that Mary Ann's maiden name was Stoneham, the daughter of Joseph Stoneham who died between 1858 and 1861 in Essa Tp, Simcoe County, and Margaret Feltis (who was still alive in 1891). On the 1861 Census, a couple of the Stoneham siblings were in the Walker household but they were also listed with their Widowed mother, Margaret.
There is a write-up about Joseph and Margaret's son, William Henry Stoneham where it says that he was born in Brampton, Ontario, in 1852 and that Joseph Stoneham, a brick layer, was a native of England and Margaret Feltis a native of Ireland. The family moved from Brampton to Simcoe County:
http://www.mocavo.co.uk/Commemorative-Biographical-Record-of...Tree on Ancestry -
http://trees.ancestry.ca/tree/59103911/person/32041221767Name: Joseph Stoneham
Marriage Date: 31 May 1839
Marriage County or District: Home District (Toronto)
Spouse: Margaret Feltis
Name: Joseph Stoneham
Event: Living
Year: 1858
Place: Simcoe County
Province: Ontario
Source: South of Sodom: The History of South Norwich, The South Norwich Historical Society, 1983.
Volume/Page: Vol.2,pg.247
1861 Ontario Census Dist. 2 - Essa Tp., Simcoe County, pg. 15
Margaret Stoneman Milliner b. Ireland 47 Widowed
Thomas Stoneman Labourer b. U. Canada 19
Margaret Stoneman b. U. Canda 15
Wm Henry Stoneman b. U. Canada 11
Ellen Stoneman b. U. Canada 9
James Stoneman b. U. Canada 7
1861 Ontario Census Dist. 1 - Mulmur Tp., Simcoe County, pg. 14
Wm Walker b. Canada 27 married
Mary A Walker 21 married
Wm? Walker b. Canada 3
Sarah Walker b. Canada 1
Margrat Stoneham b. Canada 16 (listed also above with widowed mother)
Wm H. Stoneham b. Canada 11 (listed also above with widowed mother)
1871 Ontario Census 41 - Simcoe County South, O2 - Essa Tp.
Margret Stonham 59 b. ireland Widowed
Margret Stonham 26
William H Stonham 21
James F Stonham 17
Thomas Stonham 28
Ann Stonham 26
Joseph Stonham 4
Name: Thos Stoneman
Age: 27
Birth Date: abt 1844
Birth Place: Toronto, Ontario
Marriage Date: 24 Feb 1871
Marriage County or District: Innisfil, Simcoe
Father: Joseph Stoneman
Mother: Margaret Stoneman
Spouse: Ann Quail
Spouse's Age: 22
Spouse Birth Date: abt 1849
Spouse Birth Place: Montreal, Quebec
Spouse Father: John Quail
Spouse Mother: Bridget Quail
Name: William Stoneham
Age: 22
Birth Date: abt 1850
Birth Place: Canada
Marriage Date: 31 Jan 1872
Marriage County or District: Cookstown, Simcoe
Father: Joseph Stoneham
Mother: Margaret Stoneham
Spouse: Elizabeth Switzer
Spouse's Age: 19
Spouse Birth Date: abt 1853
Spouse Birth Place: Canada
Spouse Father: John Switzer
Spouse Mother: Ann Switzer
Name: James Stoneham
Age: 22
Birth Date: abt 1854
Birth Place: Brampton, Ontario
Marriage Date: 23 Feb 1876
Marriage County or District: Essa Tp., Simcoe
Father: Joseph Stoneham
Mother: Margaret Stoneham
Spouse: Mary Jane Hamilton
Spouse's Age: 18
Spouse Birth Date: abt 1858
Spouse Birth Place: Cookstoun
Spouse Father: Robert Hamilton
Spouse Mother: Susan Hamilton
1881 Ontario Census 139- Simcoe North County, A1 - Nottawasaga Tp.
James Feltas Stoneham 26
Mary Stoneham 22
Wilford Stoneham 2
1881 Ontario Census 118- Simcoe County, C2 - Essa Tp., pg. 54
Joseph Stoneham 24 b. Ont. 24 married
Nancy Stoneham 24 b. Ont. wife
Robert Stoneham 2 b. Ont.
Joseph Stoneham 0 b. Ont.
Joseph Smith 18 b. Ont. Lodger
Margret Stoneham 87 b. Ireland Widow
Name: Wm Henry Stoneham
Gender: Male
Birth Year: abt 1853
Birth Place: Haldimand, Ontario
Age at Death: 47
Death Date: 18 Jan 1900
Death Place: York, Ontario, Canada
Name: Edward Henry Walker
Event Type: Marriage
Event Place: , Simcoe, Ontario, Canada
Gender: Male
Age: 36
Father's Name: William Walker
Mother's Name: Mary Ann Stoneham
Spouse's Name: Josephine Mabel Miller
Spouse's Gender: Female
Spouse's Age: 25
Spouse's Father's Name: John Miller
Spouse's Mother's Name: Annie Currie
Name: Mary Ann Woodwork
Event Type: Death
Event Date: 01 Dec 1923
Event Place: Stayner, Simcoe, Ontario
Gender: Female
Age: 50
Birth Date: 14 Jan 1873
Birthplace: Ontario
Burial Date: 03 Dec 1923
Father's Name: Wm Walker b. Ontario
Mother's Name: Mary Ann Stoneham b. Ontario
informant: brother Edward Walker of Stayner
burial: Stayner Cemetery
Name: Henry Edward Walker
Event Type: Death
Event Date: 26 Mar 1929
Event Place: Stayner, Simcoe, Ontario, Canada
Gender: Male
Age: 59
Birth Date: 07 Jul 1869
Birthplace: Ontario
Father's Name: Wm Walker
Mother's Name: Mary Ann Sloneham