Hmmm. I don't have access to the original records, but I have the same marriage information in St-Louis-de-Gonzague, Nashua, NH.
However, I also have an unmarried Marcelline Fontaine in the PION-dit-LAFONTAINE line with siblings Gilbert "Hubert" and Celina.Their parents are Olivier Pion-Fontaine and Agathe Touchette. Gilbert married Julia Dussault. Celina married Cleophas Lemelin. All lived in the northern VT area. I have Gilbert, Marcelline and Celina all being born in St-Hughes, Bagot, QC (without source information). Birth date for Marcelline Fontaine is 3-Sep-1850 St-Hugues, Bagot, QC. If she were a child of Jean-Baptiste Fontaine and Archange Jalbert-Carleton, her birth date would conflict with their son Joseph Fontaine b.18-Oct-1850 (approx).
So I suspect you are on right, and that Marcelline doesn't belong in Jean and Archange's family. However, we still need that original record proof.