I'm actually betting on the sister being the one who used the regression magic. After all, the one who uses it needs the jewel and won't remember anything but what if she used it in hopes of the little one having a good family and life instead of the awful one she had? She wouldn't remember but she had in the past life opportunity to get everything and do it. And with her little one having the adult brain, she would see through things that they didn't see through as kids. And wouldn't keep running and escaping from her just because she's "scary".
Who ELSE would have the motive? EVERYONE ELSE was made to be understood to be dead. If she truly hated her, she wouldn't have bothered to ask those things in the first chapter. The first timeline, neither of them communicated properly and a lot of misunderstandings along with manipulations were afoot.
