Still air in the car always sets off my motion sickness. I don't like to drive or ride in a car without the fan going, ideally at medium or higher. If you're not driving at freeway speeds, opening the window might be an option.
Strong smells and warm temperatures are bad.
She's not still rear facing, is she? If she is, you want to turn her front facing. My motion sickness is always worse when I'm facing the back of a train.
This isn't the time for the scenic route. Major highways are best. People who get carsick do not enjoy scenic drives on winding roads, trust me on this. You might be able to make that tolerable, but it's probably never going to be enjoyable for her.
We have chewy candied ginger candies here in the US. Do you have those? Those help me with nausea.
Make sure you have something on hand for her to drink. Soda with sugar works well (ginger ale or Coke). Diet soda or water will help some, but not nearly as well. Juice that is more sweet than acidic might work. You don't want milk for this- it won't help nearly as well, and milk vomit is NASTY.
Sugar does help a lot of people with nausea. Unfortunately, healthier alternatives just don't seem to work as well.
Prepare for the worst. Have towels, paper towels, some bottled water for cleanup purposes, and a change of clothes (better, a couple of changes of clothes) for her. Have plastic bags (ideally, big Ziploc bags, or some bag that you can close and make watertight) to hold things that have been vomited on. If she is old enough to vomit into a cup or bag, a supply of large disposable paper cups or Ziploc bags would be good for this. That's going to work better than a reusable basin, because then you don't need to wash it, and it's easier to get the smell of vomit out of the car as soon as possible. The smell of vomit can trigger more vomiting.