Keep trying her on the phone and maybe also check with any of her friends of acquaintances if you know their numbers. Chances are it's the bad driving conditions and the fact that it's Friday night and she's probably just gotten distracted or delayed. Do you have someone else you can call who can come over and wait with you and keep you company so you won't feel so worried or so at least you'll have someone else to help keep you grounded?

Nochowder's advice to wait until a bit later and then call the police and emergency rooms is not comforting, I know, but that's what I'd do, too. Do you by any chance know what your sister's car's license plate reads? That'll help.