Aaahh, yes--having only one, I sometimes forget about the fighting that can be prevalent among siblings sitting side-by-side.
I can relate to the motion sickness, though. Hayley started getting carsick at about 18 months old, and still does, at almost 5. Luckily she's learned to aim it well, and we bring a bowl, a gallon of water, washrags and extra clothes on every trip over 20 minutes, just in case. It used to be a huge production, she'd puke all over her clothes and carseat, I'd have to find a place to pull over, take her out, clean her up, change her clothes, wipe the carseat up as best as I could and then just get to where we were going and clean the carseat better when we got there.
Back then I didn't know any better, and machine-washed the harness straps pretty often.
Since we worked out the "carsickness kit" and Hayley learned how to aim her vomit into the bowl, things are much calmer on road trips. All I have to do is pull over and use the gallon of water to rinse out the bowl, use a washrag to clean up her face, give her a drink of water and we're on our way again. No more pukey carseats, THANK GOODNESS! We have an extra seat cover for the Futura now, just in case.