This happened to us last year flying out of MHT en route to AUS. Got a message at 8:30 am that our flight was cancelled, presumably for weather, although there never was any explanation given nor any weather issues nationwide. Our scheduled flight was in the early evening and we weren't anywhere near the airport. By the time we got a customer service agent, there were no flights for the four of us from any airport within 150 miles of MHT for the next three days.

We asked to be routed to HOU or SAT or DAL but were told we'd have to pay $$$$ or lots of miles. Finally, after speaking with a supervisor we were able to get routed to HOU that evening on SWA via BOS "as a one time courtesy." We had to return our car to MHT, rent another one, get a hotel in HOU since we wouldn't get in until 1:30 am or so, and then rent a car to get to AUS. Of course that wasn't covered...

Despite multiple requests, no explanation was ever given to our inquiries of what was and was not a sister airport (the agent called it "code-sharing") nor was there any reference to what specific policy prevented this change.

I love SWA and travel them almost exclusively, but man, that was a major PITA with two little kids at the end of a two week trip.