Benadryl liquid. Ask your doctor how much she should take. You need to give it to her about 30 minutes before you get in the car.
Adding on:
We have lots of carsickness experience in our family. She will tend to get sick in ANY kind of moving vehicle or ride (like at an amusement park). I've even gotten sick at a park with the kind of merry-go-round that you push yourself, and my grandmother was unable to swing in a swing at ALL. Heat is a contributing factor. Also, don't let her look at books in the car. One more thing that would be funny if it weren't such a pain. It's absolutely possible to get sick at the MOVIES watching something moving on the screen! Polar Express and Cars almost did me in, and trains on the screen can too, even though I'm sitting still! And it's possible to watch a real-life train moving in front of you and get sick from that!
Probably it will get better as she gets older. The Benadryl will make her sleepy, but that's OK when you're traveling. When she is older, she will be able to take Bonine, which is a chewable tablet that won't make her sleepy and it lasts 24 hours. The Benadryl only lasts for six. (Store brands are fine.)