The cited Nellis–Nelles books state that Mary and Arthur were married and had a child Arthur, but does not give when the elder Arthur died.
Find a Grave shows that he died early, in 1862 at age 16.
Pretty young to be married and have a son.
Find a Grave shows that Arthur's brother John Nellis (also?) married Mary and that the younger Arthur is John's son with Mary.
This raises doubt in my mind that the elder Arthur both married Mary and had a son with her.
Sources
"Canada, Prairie Provinces Census, 1916," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:KMPW-95W : 23 February 2021), Mary Tisdale in household of Arthur A Tisdale, Manitoba, Canada; citing p. 12, line 37; Library and Archives Canada, Ottawa, Ontario; FHL microfilm 2,434,949.
"Canada, Prairie Provinces Census, 1926," database with images, FamilySearch (https://www.familysearch.org/ark:/61903/1:1:QPJY-PZSY : 24 September 2020), Mary Tisdale, Winnipeg, Manitoba, Canada; citing Census, Winnipeg, Manitoba, Canada, Library and Archives of Canada, Ottawa.
Find a Grave, database and images (https://www.findagrave.com/memorial/123089297/mary-tisdale : accessed 06 September 2021), memorial page for Mary Acheson Tisdale (8 Oct 1844–18 Aug 1938), Find A Grave: Memorial #123089297, citing Elmwood Cemetery, Winnipeg, Greater Winnipeg, Manitoba, Canada ; Maintained by Christina (contributor 46772328) .
Nellis-Nelles Immigrants From the Palatinate 1710, published by The Nellis and Nelles Family Associations and the Herkimer County Historical Society #2516: Vol. 1, p. 381; Vol. 2, page 67
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