How To Change Angst Into A Feel-Good

I got reincarnated! Into the prettiest, kindest, daintiest, most perfect female leads little sister. The problem is that this story is an angsty R19 story. The male lead is an insane bastard with a streak of madness and my sister is doomed to be a mental wreck due to his obsession, facing a tragic ending where she loses her mind. I absolutely cannot let my beautiful and kind older sister meet such a miserable fate! From now on, Im in charge of the genrefrom an angst to a feel-good story! Ill be fine. Im not Astella. Im not as pretty as her so the male lead would never fall for me. Cute. That isnt the word that should be coming from the cruel and cold-blooded male leads mouth Ill be fine, right? I dont have Astellas delicate wrist nor her small waist that could be encircled in one arm. Not to mention, her fair skin. Him falling for me would be out of the question. You cant escape. Theres no place for you to escape. Surely Id be fine? Our first encounter wasnt as passionate as it shouldve been in the story. Plus, the male lead wasnt as insane as he was described to be. So, I should be fine, right? How should I explain this urge to strangle the bastard who touched you? I dont think Im fine! --- 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.