Answered on Feb 13th, 2013 at 9:12 AM
Warrants don't ever expire. So, the police can arrest you for a warrant no matter how old it is. If they pick you up on a warrant and you bond out, you still have to appear in court at some point for the underlying charges. If you don't appear in court or pay the fine, then the court issues another warrant. In this situation, it sounds like the police pulled over the car, looked up your sister's driver's license identification and discovered she has a warrant, so they could have arrested her just for that. But having found marijuana on her gave them another reason to arrest her.