All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
We advise you, first and foremost, to earnestly advise your sister and clarify that the basic principle is that it is not permissible for a woman to travel except with her husband or a mahram (a close male relative whom she cannot marry). She is obligated to adhere to modesty (covering herself) and wear the hijab, whether she is traveling or not. Let this advice be gentle and kind, as it may be a means for her righteousness, uprightness, and religious commitment.
If she responds and refrains from traveling or if her journey can be done in a manner in accordance with the religious guidelines, then praise be to Allah. However, if it appears that she intends to travel in a way that goes against obedience to Allah, it appears - and Allah knows best - that there is no objection for you to accompany her, take her to the station, and not leave her alone with the driver in order to minimize the evil. Scholars have mentioned that forbidding evil has different levels. The first level is that the evil is eliminated and replaced with its opposite (something good). The second level is that the evil is reduced, even if it is not entirely eliminated... and so on.
Allah knows best.