
This is still possible because we don't know what happened after the MC died. It seems like her father was the one who chose her to have memories of the past but we don't know why yet, maybe it was guilt over how he treated his daughter, maybe it was something else. What we do know from the diary though, is that when a younger sister is born, the younger own awakens to her powers first and the older one doesn't receive her powers until she is touched by the younger after the awakening. The MC's sister could still be a demon but, at the very least, she has proven to be her younger sister with this.
Can someone explain it? So the owner of the diary is also a spirit user same goes to her sister. But what is it link those with resonance and cosette??