I think Jack has split his personality because he can't protect his younger siblings from grief excessively. The timeline is that Jack reported his father and the family ran away with 10,000 pounds. When his father came to the door after breaking out of prison, Jack locked his younger siblings and went to negotiate with his father, but was pushed off the cliff. When he woke up, he returned to the house and found that his siblings were all dead. At this time his personality split. Put the third sibling in that room, build a wall to seal up, and then split the personality of the siblings to continue living.
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