Update
Cousin reports to me that he is extremely pleased with how an AC Concierge Agent at YUL stuck with the file, and was able to get the other cousin an aisle seat. Everyone is now satisfied.
Discussing this route reminds me that when I first flew it back in 1998, it was operated with a 767...and was pretty full. I would figure at least a A321 would be appropriate for the seasonal snowbird migration, and the fact that it is a hub for a *A carrier, US.