oh, this is not a translation issue but rather a cultural one. The sister can't come, problem solved. The Thai won't understand that you bother yourself with plans that were changed/cancelled. In your case, a Thai would have asked "She can't come, at what time do we go then?"
Everybody can renew their driving license all around Thailand, Thai and foreigners alike, wherever their address is. Many Thai don't live at their registered address and transport department decided to take that into account some years ago, thank to their internet linked system.