Changing the Genre From Angst to Heartwarming

I've been reincarnated. Into a novel I used to enjoy in my past life, no less! My name is Renatis, and I'll be playing the part of little sister to the female lead in this dark, tragic whirlwind of a story where a beastly bad boy shatters my sister's hopes, dreams, and her life. My sister is too good and too beautiful, and I'm not about to let that happen to my perfect sister. So, I'm taking charge. I shall turn this dark, tragic story into a heartwarming tale of healing. I will save my sister.(Source: WEBTOON)