So here's the theory,
Spoiler!
the sister remembers the past life and is actually the one who turned back time. Idk
Comment by Chikita on The Sister Who Once Hated Me Now Loves Me - Chapter 11
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@Chikita
Spoiler!
while i agree that she turned back time, the book said they lose their memory. the book never said anything about losing your feelings about others at the time of time travel ; postmortum, the sister probably regretted things and wished things to be better between them.