Indeed you can thank your ancestors that someone had the foresight to save those letters and pass them on to you Julia!
Stanbridge is the community that you are looking for and the parishes there were erected in 1846 and Stanbridge East in 1952. The county marriage register for Missisiquoi recorded the locality of the marriage as "Stanbridge".I went into my cd of the area as my Grenier family was from this county.Here I found the marriage of Abraham Godin to "Desanges" Marcoux, 5 Feb 1855, Stanbridge, son of Louis and Francoise Brault.Going to the Marcoux listing I found Desanges was the daughter of Jean Marie and Desanges Touchette.....Now I can look further for the ascendance.Rosalie may have been used to distinguish her from her mother??
Indeed I can somewhat verify the writings that you are working from as I also found this information:
Marie Marcoux (prob "Marry") married Francois Regis Leblanc on Jan 20, 1863
Philomene Marcoux married Isodore Pratte (with an "e" herein) on November 24, 1856
and the second (??)marriage of Jean Marie Marcoux widower of Desanges Touchette to "Domithilde" Tremblay, May 27, 1851
all these noted as at Stanbridge, Missisquoi, Quebec.
As you can see the names of the father, brother-in-law and the sister seem to match what you have in your written document.