What do you think of me telling a girl she needs to leave her sister in the car next time she gets cigarettes?
Do you think it's okay for her to leave her sister in the car?
Yes, it is not going to be for very long anyway.
Telling others how to behave with their siblings can be controlling. She may feel like you're trying to push her around and could get upset at you if you're too bossy. At least have a good reason for asking for this request before you do it.
So if I have a good reason, you are saying I will get a good response in return?
I think you will get a surprising response. Unless her sister is being harmed, it's not okay to tell someone what to do.