Sorry it's been so long since you posted! I've only recently started watching this board.
I found why your searches were not successful. The indices to the Drouin Collection have been built from the marginal notes of the parish registers they contain. For some reason, the marginal note is missing from your 3rd great grandfather's marriage entry. You can find it here:http://content.ancestry.com/Browse/view.aspx?dbid=1091&p...
If that link doesn't work for some reason, email me, and I can walk you through how to get to it.
That page shows the only marriage that occurred at the parish church of St-Nicolas on 23 Oct 1821. The names are slightly different than what you've mentioned. They are Michel Lemieux and Angélique Magnan(sp? - difficult legibility). The "Quebec Vital and Church Records (Drouin Collection), 1621-1967" collection will be your friend in trying to track down the rest of the family.
One problem you'll have to overcome is that you appear to only have English names. If the above record is the correct one, Michel becoming Mitchell seems fairly clear. However, "Austin" is nothing close to any French name that I know. (I suppose, perhaps, "Augustin".) If you have any idea what the original French names were, it would be very helpful. That said, the brothers were probably Michel et Jean before Anglicisation.
Well, I hope at least that first link to the mariage of Michel and Angélique helps!