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.

Because the prince was heading to the place with sun church. Those guys hate the dark elves, who have and use dead mana as it's seen opposite of light. His disguise uses dark mana so going to the main location of where the sun church worshippers are had a huge risk of the light exposing his disguise and revealing he had dark elf blood. The bracelet originally was there to strenghten his disguise while he's practically surrounded by people who want dark elves dead. The blessing on the other hand from Cage strenghtened it further, making it more difficult for sun church's blessings and power to remove his disguise.

Without the bracelet, the prince would most likely have been executed after being busted for his disguise coming off and for having dark elf blood, even if it was just a quarter of the blood. It's all mentioned in previous chapters.