I would try a lot more smaller solutions before I resorted to forward facing.
First off, position the seat so that she can't see out of the side window, because that will be the one provoking the sickness. If you can't block it out, then try some of the sunshades that at least limit the view.
Second, try to position the seat so that either she can properly see out of the back window, or can't see the back window at all (depends on the height of the car, size of seat etc.). The former is preferable, because it will enable her to have a fixed point of reference to focus on. You can take the headrest off the rear seat for example.
Third, try to keep her cool and, when you can, time journeys so that she's neither hungry nor full.
I would also consider getting a travel bib/ overall so that any vomit just gets on that, which is popped in the wash after every journey. That was a massive help for us!
Remember that the safest seat in the car is the rear middle. And luckily it's less likely to bring on sickness than either of the rear sides seats - because of being further from the side windows. If you have space (you mentioned another car seat), that would be a good place to move. Front passenger seat rear facing is an option, but it is statistically much more likely to be damaged in a collision than anywhere in the back.