Getting to Waterton
You could fly and clear customs at Del Bonita (a grass strip right on the border also known as Whetstone International ) Then a short hop to Cardston and have your relatives pick you up at the Cardston airport. Make sure you tie your plane down well. It is less than a half hour drive from there to Waterton. I don't believe there are rental cars in Cardston.
If you choose to do this, stop by the Remington Carriage museum in Cardston. They have a wonderful collection of horse drawn transportation. Worth the stop.
Alternatively drive the "going to the sun" road through Glacier Park, take your time and enjoy the scenic trip.