Unacceptable behavior such as hitting, biting and kicking need to be nipped in the bud ASAP. It wasn't and now your grandson thinks it's a joke. A barrier would be a joke as well, he'd think ~ "I have to get over this or through it so I can kick!" so it wouldn't be a solution since the problem is HIS behavior. Mom and anyone witnessing his kicking will have to work extra hard to help him get this under control.
~ He needs to be told that his behavior is unacceptable and shown other ways to express his feelings verbally. Is he tired or hungry? His feelings should be acknowledged, but the kicking not excused.
~ He needs to have logical consequences for his behavior immediately and clear limits, "We do NOT kick, it is never acceptable." I know they're in a rural area, so does Mom have the option of simply stopping the car, and removing him from the car seat and the car? (Sister staying in her seat.) He should know instantly when he's done something wrong, so removing him from the situation for a brief time-out, a couple of minutes, would be a good way to let him cool down, and after a while he'll connect his behavior with the consequence and figure out that if he kicks he's removed from the car.
~ An additional consequence once he's home following any kicking will reinforce that what he did was wrong, too. No TV the rest of the day, or something he was looking forward to, no playing with a "special" toy, etc. Since you've already indicated that taking his cars away did no good you know a consequence is meaningless if a child doesn't care, so parents have to look at THEIR child to decide what will work as an effective consequence, sometimes by trial and error. *Also, IF his sister is teasing him or provoking him in any way she will need to have consequences and be disciplined as well.
~ Each kicking episode should be dealt with consistently, the same way as the last time, each and every time. The predictable response ("Okay, you kicked again — that means time-out") will set up a pattern that he'll recognize and come to expect. Eventually, it will sink in that if he kicks, he'll get a time-out (or other discipline that has been established.)
~ When he's settled down he should be asked why he kicked his sister, and once he's answered told it's OK to get angry, it's NOT OK to kick, and that he needs to find a new way to show his anger with his words. He also needs to taught he needs to tell his sister he's sorry after any kicks. Yes, he won't be at first, but as incidents occur over time he will learn it's wrong to hurt another person and will hopefully feel compassion. And, his sister should say, "Ouch! You hurt me!" rather than calling him a baby when he kicks. Maybe he wants to be a baby and he likes hearing that? Young children think differently than us sometimes, and what we think is a good thing may be the opposite of what they think! ; )
~ If he gets angry because he's hungry Mom can make sure he eats before traveling and carry easy-to-eat snacks like cereal, crackers and juice or water. If he's tired she can bring a blanket and something he likes to cuddle with in bed. If he's bored, she can bring books and quiet, non-aggressive toys for him to play with, which she can take away if he kicks.
~ Any good car trips without kicking need to be rewarded with praise once home, and immediately when he verbalizes his anger or frustration, and over time he's realize just how powerful his words are.
~ His TV time should be closely monitored. Some cartoons and other shows can be filled with shouting, threats, shoving, hitting and kicking, and should be quickly eliminated from his viewing. An adult should watch TV with him, and talk to him about situations that arise: "That wasn't a very good way for him to get what he wanted, was it?"
His diet should be scrutinized as well. Junk food, sugar, chocolate and caffeine should be eliminated as they can all contribute to unruly behavior.
And, sometimes a child's aggression requires more intervention than a parent can provide. If your grandson continues to kick and consistent efforts to curb it have little effect, Mom will need to talk to his doctor, who may in turn recommend a counselor or child psychologist. That way they can determine the source of the behavior and help him through it.