My theory is that her sister was the one who turned back time

I think so too, maybe she never hated her in the first place?

She did hate her. She hated Harper’s mother for taking her mother’s place (The duke and his first wife are divorced). Apparently she turned back time cause she was bored with life. Even if she didn’t turn back time, it was clear from the get go that the sister hated Harper, she literally had no qualms with killing a literally child.

So why is she different this time? Very tsundere vibes