I was angling towards #3 being the sister but after this episode with her seeing Boone while he was on the phone to his sister and genuinely thinking he was a ghost, it can't be her. I'm sure it's Hester (who is still alive) now.
Also Boone mentioned being trained to slow his heart rate enough to fool even a coroner into thinking he's dead, his twin sister would likely be able to do the same thing (and if you can fool a coroner you can fool the Chanels ).
).
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