your friends are certainly not what i would expect friends to be, they should not have left you alone waiting in a car whilst they were inside having fun. they were not copnsidering your safety or feelings at all.
it seems to me as though they are just using you, please learn to say NO, you have no responsibility to either of these people, and in all reality your sister (i am guessing she is older than you) should be the one looking after you, taking you places rather than the other way round. where, when and why you go where you choose to go is none of their buisness, you do not have to check in with your sister over your movements if you choose not to, you are an adult, and with that comes the rights of freedom, thought and choice. if you want to schoufer your sis around then tell her you will only do it if she pays for the petrol and parking before you set off, if she refuses then tell her to get a cab and then you drive away!
as for transferring, if you are hapy at the school you are at appart from these two upsetting you, then it seems daft to transfer, would it be possible to talk to your mentor/tutor/scool councillor type person about you swapping residence so you are not sharing with these two. or would a chat with your parents help, they may be able to talk to your sis and get her to stop using you for her benefit or help you find some other residence for next year.

just remember you can choose who to keep as a friend, you can't choose your family but you can choose the contact you have with them.