My interpretation of the ending was that her sister could also see auras, which is why she was able to find the matchbox. The cigarette butt in the matchbox was the reference point for the aura - with it, someone can identify the rebellion's leader and his messages. The sister mistrusted Nasrin, especially when Nasrin lied to her about the aura early on. She went to join the resistance, and her father was pleased.
What I didn't get was how the father knew to look under the bed.
It may be that the whole thing was a ruse by the resistance at Nasrin's expense - they knew Nasrin was going to betray them, so they planted fake messages and set her sister to watch her.
I did feel this story was far too vague about the actual content of the messages. It seemed to place so much value upon them that they needed this whole aura-based encryption method to proceed, but the story wanted so much to focus on Nasrin's emotional state that it sort of ended up giving short shrift to the world she inhabits, and I don't think that worked for the story.