An ordinary university student reincarnated into the novel "Eternal Love", written by her at the age of 15. However, instead of arriving when the original story began, she reincarnated seven years prior to the events of the novel as Ran Romea de Lazia, the daughter of the male lead's stepmother - a character that she didn't even know the name of before reincarnating. Originally, the stepmother tormented the male lead, Yustaf, so much that he began to distrust other women, so Ran tries to change the events that are going to happen, yet their parents and little brother still die. With Yustaf away at the academy and his uncle in control of the family estate, Ran has to work as a housekeeper for two years and begins working on the "Male Lead Happiness Plan" where she will join him along with the female protagonist and let him have his chance at happiness.
Suddenly waking up as the antagonistic side character (with a bad end) in a novel you wrote when you were younger (and cringier) is probably any author's worst nightmare. Being unable to do very much to stop most of your family from dying isn't much better. Throw in a little 'not-related-by-blood' one-sided insest interest and other-sided obliviousness and you have something that seems more classic of Japan than Korea but, eh. It's still isekai. Story is about the protaganist's struggles to support her story's 'main character' using the 'cheat' of her knowledge and the eventual love interest which is so very obviously her step-brother... but as she was isekai'd and barely spent any time being 'raised' with him before he was sent off to school (like... a few days?) it's less 'whatcha doin step-bro?' and more just general isekai shenaniganry.