The 1910 US Census indicates that William immigrated in 1866 and Isabel in 1868, which suggests a marriage in Minnesota or elsewhere in the US. Do the death certificates give no clues as to their birthplace or the names of the parents.The census also indicates their birthplace as Canada-Fr. which suggests Quebec or perhaps New Brunswick which had a large French-speaking population.
The 1875 Minnesota State Census shows Isabel's birth place as Fort Garry (now Thunder Bay, Ontario), but her name is given as Elizabeth.William must come from elsewhere or he would have been identified as Fort Garry as well.
The 1885 State Census shows that the oldest child was born in the Dakota Territory, not Minnesota, so perhaps that is where you need to be looking for a marriage. Isabel is also noted as Lizzie in this census.
1905 State Census indicates Isabel as "Ind(ian)" vs. White. 1880 Census shows her as Indian and the children as 1/2 white.