Eyy I know this. I can't remember the name right now but I'll reply again if I do.
Some more details to this is FL and her sister are half-siblings iirc (and she was probs an illegitimate child) FL was the most beloved child of the emperor and most powerful too except for her black haired sister. Unknown to FL, the black haired sister was being tormented in the palace by their other siblings and their father turned a blind eye to this. Eventually FL's sister succumbed to darkness and destroyed the royal family, killed everyone and she died too. It was then she blamed FL why couldn't FL have protected her before.
FL returns back to the past and is now hell bent on protecting and caring for her sister. She also finally realized that her perfect and kind family weren't so kind (especially her father who doted on her the most but turned a blind eye to how his youngest daughter was tormented)
It's "I Raised My Younger Sister Beautifully"
Link: https://www.mangago.zone/read-manga/i_raised_my_younger_sister_beautifully/
Anyone know that 1 Manga, where the Fl has blonde hair and her sister has black hair. The sister with black hair has the ability of a snake power and that it was considered evil. The blonde hair girl was like lights. Are healing and that the black hair Sister was mad at her for not saving her when she was being tormented They both die, but the last moment, the blonde hair sister said and the next life. I hope that you will be my sister and I will be kind to you. I think that's how it went