I have been researching Sophie, the daughter of Joseph Fortin and Elizabeth Champagne for a few years, as the only child of the family I wasn't able to find anything about. It was just recently I started looking at this as a possibility, and then I stumbled upon this discussion. I have found good reason for Sophie, the spouse of Louis, to be the daughter of Joseph and Elizabeth (I'll add information to her page as research notes, but will summarize here).
Joseph and Elizabeth were in Farmington in 1860. They didn't stay long as they were back in Quebec for the 1861 census, but they were there using the surname Foster (and Elizabeth was listed as Isabel). I am certain this is the same family based on the names of the children, and these younger children returned to Maine and continued using Foster as their surname. As well, Frederick Fournier, Sophie's son, in his World War I registration lists Levi Foster as his relative (this would be his maternal uncle, who was born Olivier Fortin). In later census records he is living in Gardiner with Mary and Luther Buckmore, Mary being his cousin through the Fortin family. Mary Bickford is listed as his cousin in his record at the military home in Augusta, but I think this was a clerical error and was likely Mary Buckmore.