Then she was trolling for attention from her parents. Saying things openly which are contrary to your parents values is making a statement.
Either they were ambivilent to certain types of theft, or she was challenging someone to tell her parents that she disrespected them. If it was me, I would have obliged her, or at least confronted her on whether they had taught her that stealing was ok.
The parents were't home at the time. I should have been more clear about that to start with. The sister apparently thought I was "cool". Wrong!