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 incest 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.