Hi. Congrats on your expecting twins!!
We don't have a people mover either, but when we were expecting our twins, we moved DS out of the baby seat that was on the drivers side, and put him in a maxi rider in the middle. We then had the baby capsules behind the driver and passenger seats.
DS was 3 when the girls arrived, so a bit older than yours, but he would happily crawl under the baby capsules and into his seat in the middle. We called it a "big boy seat", so it was exciting for him. We also did this a couple of months before the girls arrived, so he didn't feel like the new babies were pushing him out.
With your car, I think it would be easier to have the babies in the second row, because it will be easier getting them in and out. You DD will quickly adapt to being the "big girl" in the back row!

Maybe give her a trial run in the back and see how she likes it. Make a game of it, and tell her how special and lucky she is to be able to sit back there.
Good luck with it, and also your new arrivals when they come.