Oshi no Imouto ni Tensei shiteshimatta node, Saikyou no Akujo ni Natte Oniisama wo Dokusen suru Koto ni Itashimasu

Amamiya Nagisa was very much in love with the evil marquis Neun, a character from her favourite light novel. On the verge of her death, what awaited her on the other side wasn't oblivion, but being reincarnated into Neun's younger sister Frau! They might not be related by blood, but in the world of the novel, love between siblings is horribly taboo. Unable to bear the thought of anyone else having her beloved Neun, she will become the villainess Frau, tear down all his romance flags, and give them to him herself!

(Source: Akita Shoten, translated)