GUYS! The spoilers below are misunderstandings!! Here’s what really happened:
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The world she’s in right now is a script or novel that she read that her sister or sister’s fiancée gave her, this was mentioned on the earlier chapters that she received gifts from one of them. The part where the mom, king and crown prince all died are what is “supposed” to happen in the original had she not transmigrated and didn’t demote the 2nd prince and his mother. She only realized this after the crown prince dies and was able to prevent the subsequent deaths. Her mother only pretended to drink the poison that is supposed to slowly kill her but is actually a ploy to stay safe from more dangerous methods of killing her. In the end, only the crown prince (my fave character) was killed when it was supposed to be the three of them.