I would eliminate using AA miles from the running, as it looks like you would need to connect through London on British Airways and their fuel surcharges are high.
If you use United miles, you can make one connection in Newark to get to Oslo. Other cities in Norway will require connecting in Europe, probably on Lufthansa in Frankfurt, adding another connection.
If you use Delta miles, you can fly direct to Amsterdam on Delta and connect on KLM to Norway. You can also connect on Air France out of Paris, but I don't see any direct flights from MSP to Paris. KLM also has flights to Bergen from Amsterdam, I didn't check any other flights in Norway.
I would start searching for flights using both the Delta and UA search engines, and see which schedules work best for you and your sisters. When you are going that far, minimum travel time and connections will be big factors.