I have not been able to find marriage records for children of the marriage of Édouard and Marie-Louise Potvin.
From the Édouard's marriage to Alphonsine Potvin, I have found marriage records for
François to Merilda Gingras 25 Apr 1904 at Grand-Mère
Georges to Mélenda Carrier, 26 Jan 1909, St-TIte.
Édouard first got married at Roberval, quite a distance north of Charlevoix where you have found a marie Lalancette in the 1881 Census.In this regard I have found a marriage record for Ulric lalancette (son of Michel); that is probably the Ulice(brother of Marie) that you have found in the same census.
Your best deal is to try to find siblings of Marie who were married in Lowell MA to determine her parents' names..My Gen .Soc, only has the marriage repert. of St-Jean-Baptiste for Lowell.