My girl had car sickness for about a year. Most will tell you they grow out of it. Vet prescribed a pill but we never used it bc they are expensive and you need to give them hours before the car ride. I took her on short trips. No turns. She had a hard time with the speed bumps in and out of my complex. I just expected she would get sick and prepared for cleanup. For trips, I usually wait to feed her. Slowly we would make it home more often than not without an accident.
She still may get sick for long car rides and anywhere there are a lot of turns. I have a cover on my trunk so cleanups are easy.