Stories from patients and results from studies don't agree. There's a wide range of reports from "miracle cure" to "symptoms worse than ever." No one has died from this treatment. The former Surgeon General C. Everett Koop is one of those patients who has been helped by prolotherapy--so much so that he began to use it on some of his patients.
A recent study from the University of Queensland in Australia report their findings. They put 110 subjects with chronic back pain into two groups. They either did or didn't get an injection with glucose and a local anesthetic and were given exercises or told to do their normal activities. Everyone got better with the injections. The exercise didn't seem to matter.
No one has done a study on the effect of driving long distances after prolotherapy. It makes sense that long periods of sitting or inactivity may be harmful, but we don't have any real data on that one. It may be best if your sister takes frequent breaks and gets out of the car to walk at least every two hours.