sounds like a battery or alternator issue. If you have jumper cables, see if you can jump start the car from your sisters car battery. other wise you can swap the batteries and see. I assume both are petrol cars. If you can start the car that way, find a volt meter, turn on you a/c. head lights etc and quickly check the voltage coming from the alternator is within range (14v ~ 14.5v) if its below that it could be an alternator issue. It also could be a simple case like loose terminals. Try searching the forum, this issue has been discussed before.