score:4
This depends a lot of your frame of reference. The road to Milford Sound is a mountain road going over a fairly high pass, so it is windy by nature. This being said, it's a well build two lane road and fairly easy to maneuver. It's a lot better than some mountain roads in Tuscany and a lot worse than driving through Texas.
My wife is prone to car sickness and she did not had any problems on that stretch. She is more susceptible to abrupt starting and stopping and "yanking", i.e very quick change of direction. That's less of issue on the Milford Sound road.
If you can give examples of roads that you have trouble with, we may be able to advise better.