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 ask Allah, The Almighty, to bless your grandparent with recovery, grant them the reward, wellness, forgiveness of sins and raise their rank. May Allah reward you generously for your keenness to adhere to the rulings of the Sharee‘ah and ask about what may seem confusing, in compliance with the verse that reads (what means): {So, ask the people of the message if you do not know.} [Quran 16:43]
If your parent commands you to drive these girls, and there is no harm incurred on you because of it, then it is obligatory on you to obey him/her. Scholars stated that it is obligatory on a child to obey his parents in what benefits them and does not harm him. We have previously underlined this in Fatwa 84942. Their non-adherence to the Hijaab does not justify your refraining from obeying your parent in this regard. Rather, you should seize the opportunity to give them advice kindly and clarify to them the gravity of sins in general and that of non-adherence to Hijaab in particular. You can also give them advice in an indirect way – which may be more effective – by having them listen, while riding with you in the car, to a sermon delivered by a scholar or preacher with guiding influence, and hopefully they would benefit from it.
In general, we advise you to be kind to your parents and to invite them to Allah gently. Kindness is the principle manner of Da‘wah (calling to Allah). Allah, The Exalted, Says (what means): {Invite to the way of your Lord with wisdom and good instruction, and argue with them in a way that is best. Indeed, your Lord is most knowing of who has strayed from His way, and He is most knowing of who is [rightly] guided.} [Quran 16:125]
It was narrated on the authority of ‘Aa’ishah that the Prophet said: "Whenever kindness is present in anything, it adorns it, and whenever it is withdrawn from something, it leaves it defective." [Muslim]
Allah Knows best.