Argh sympathies. I got terribly travel sick as a child, I used to utterly dread car journeys and the taste of Joy Rides still gives me a horrible shuddering sensation - I can't eat any synthetic raspberry flavour thanks to them!
In the front is usually the best despite difficulty for clean up, because the sensation in the back when the car turns at all is amplified. If you want to do this, for safety - push the seat as far back as it goes and disable the airbag. Or failing that, the middle in the back is often good, so she has a clear view through to the horizon. If nobody is sitting in the front seat, you could also put her on the rear passenger side and remove the front seat headrest.
Encourage her to look straight ahead and not look at things inside the car. Games like I-spy (colours if she doesn't know letter sounds yet) or looking for letters on car numberplates help with this. I also used to find that singing helped me? I don't know why this is, maybe it was calming.
Keep the temperature inside the car very cool - slightly uncomfortably cool (everyone else will probably need a jumper). Fiddle around with air con/fan vents to the back if she's in the back, to make sure they are reaching her.
Try blocking the view from the side windows, if she's in the back to one side. It helps avoid that "things rushing past" thing.
Avoid dairy or other fatty foods before travelling. All the morning sickness cures - take ginger biscuits and salt and vinegar crisps as car snacks, plain crackers like tuc or water biscuits, or maybe some cooled peppermint tea (add sugar to taste) to sip on. Cold water as well - freeze a bottle so it slowly defrosts and stays cool. You'll need a cloth to wrap it in for the consendation. I would say mints, but she's a bit young to suck on a hard sweet esp while strapped into a car seat. Maybe tictacs? Since they would more likely be accidentally swallowed than block the airway. Or some of those dissolving breath strip things, if they aren't too strong for her. Peppermint essential oil to smell might also help, but mostly I'd try to avoid strong smells inside the car - if you have air fresheners, remove them. If the seats smell of dog or smoke or anything like that, try and clean the car before you go (but not with strongly perfumed upholstery shampoo).
Stick to straight roads, motorways as much as possible. Avoid winding country/mountain roads, even if it means a longer route. The driver should also avoid lots of rapid changes in acceleration - drive in a very smooth, eco friendly style, and if you have an automatic that you can set to different modes, choose eco and especially not sport!
Take breaks where you can - it can help her stomach to settle a bit before going on. This is just a bit trial and error, especially with her age you might find there's a point that with too many breaks she gets even more upset about getting back in the car. If you can get to know the early signs before she spews (I used to get hiccups, burping is also a red flag, DS3 gets very quiet and pale) and take a break ASAP you might avoid some of the sickness. Obviously don't stop on the hard shoulder - but keep an eye on when services are coming up or come off the motorway into a town or side road. If in doubt, stop, don't push through. Sometimes you have to plan a LOT longer for the journey - e.g. if it's 4 hours of driving time, plan 8-10 hours for the travel in total.
The travel bands come in different sizes, you can get them for younger children. Amazon ones from the weird nonsense brands tend to be tiny although sometimes you get mixed sizes though, which doesn't help.
See if you can get some carpet/upholstery cleaner spray for pet messes - it's more likely to remove the smell of vomit.
If she's in a 5 point harness seat, there is an old wives' tale about sitting on a piece of newspaper - this might be worth a try for the placebo effect. I would not do it if she's in a high back booster though since it would make it more slippy between her clothing and the seat cover, which could be a risk in an accident.
Long term consider getting her eyes tested - travel sickness can be related to shortsightedness, apparently. I started wearing glasses when I was 12 but I probably needed them earlier. Glasses didn't help my travel sickness unfortunately, it's just more of a related thing.