I definitely think it was either Tai or Van. Not only because she and Van were at the back door, obviously already awake before the fire started, but because of Vans vision/dream when she was in the gas cave. Her vision tells us she knows who is responsible (why she's staring into the flames) for starting the fire. Then the arms start to grab at at keeping her stuck in the chair as fires starts to consume the cabin. This goes back to episode 2 when she was stuck in the plane seat as the plane started fire. So either other Tai started the fire and Vans dream is proof of her guilt for knowing/lying about Tai starting it or her guilt over starting it herself.