In my opinion...I think it's because the fl had seen people adoring her sis and she unconsciously wanted to be like her and the dream about the previous emperor's eye guy saying there's more about it than just seeing past and future..
So maybeeee there's a connection between these two things tht made ts possible
Yes, we've already been told her emperor's eye-gift is *more* than just seeing the past and future--though I disagree that she's *in* her sister's body "because the fl had seen people adoring her sis and she unconsciously wanted to be like her."
This set-up feels more like her "sister" inserted herself into the FL's family and manipulated the emperor to alienate his two legitimate children...
I believe her power is mind control
Similar reasoning is why I said this aspect of the story felt like the author may be leaning into a (Disney's) "The Little Mermaid"-type plot...
Sorry meant to reply to another person and I couldn’t delete
So many questions...
Why is FL in another body?
Why did FL fall from the sky (into the ML's arms, in the middle of the Demonic Plains?)
Why does said "body" look so much like her sister's? (Or more accurately, how we ~imagine~ FL's sister would look, as a child.)
Which raises the question of why the FL's sister doesn't look similar to FL, her brother, and her father?)
Why is the FL's ~apparently~ sister's body is dressed as a "dirty peasant," not the pampered princess we know her to be?
--Yes, it's obviously so we can have the conflict of the ML misidentifying the (evil) sister as his first love.
As mentioned by a few others here, I'm also now suspecting a Little Mermaid (Disney's movie, *not* Hans Christian Anderson's story)-type plot...
And that kind of storyline would be agonizing for me to follow in piecemeal increments.
（￣へ￣）