This is my little theory to why i think it was Charlotte that was hidden behind Sun in the cabin. In the episode this place is death when Charlotte is dying and she spits out all that Korean to Jin, she tells him don't let her come back this place is death. Also when she was dying it seemed like she was flashing on her own going back in time as she says why cant daddy come with us? so the possibility that she flashed forward is there as well. So i am making the connection that it was Charlotte in the cabin because she knows of the long Journey ahead of Sun that Christian tells her about.