Sorry to hear about your rotten luck.
Unfortunately, to get this resolved you're going to be riding on goodwill from the canadian dealership or Nissan Canada. I haven't worked in that industry for years (since before going back to school) but here's what I can tell you...
The dealerships liability is a bit of a grey area. Lots of dealers will buy vehicles unseen through satellite and computer auctions; to prove them liable you will need to prove that they KNEW the car had been in an accident and covered it up or that they didn't do an appropriate level of due diligence to find out whether it had been or not. If the carfax report showed nothing, and your own nephew passed the car I don't think you're going to have a leg to stand on in court. Dealerships are not obliged to spend thousands discovering every little problem a used car might have, and quite often if it looks good and starts nicely it just goes on the lot with a very cursory inspection. Odds are, they got screwed as well.
Nissan Canada may not be inclined to help since you bought an American car and a used one at that. The actual motor companies typically only care about new retail customers in their retail area. Think of it as buying a Samsung television from a pawnshop, Samsung really won't care if you have problems with the used TV. They might surprise you, but I'd bet that you'll get a nice email explaining that used cars is not really part of Nissan's business model.
Check this link out
http://www.mvsabc.com/ , these guys seem to be in charge of overseeing the car industry in BC and might be able to help you out.
Coralgurl: Dealerships are allowed to sell cars as Demo's. Usually low mileage (Think under $10,000). It's still registered as a "new" vehicle but doesn't qualify for any incentive programs. These are typically last years model that didn't get sold in time for 0% financing or other fancy deals so they put some miles on them and offer them as a heavily discounted almost new vehicles. They can be a REALLY good deal.