Migawari Akuyaku Reijo Tenseisaki de Yami no Maou ni Hoshigarareteimasu

Once a corporate slave, she collapses from fatigue one night and is reincarnated into her favorite AO―rated dating game... Instead of being the heroine Leah, though, she ends up becoming her evil older sister Letitia! The game progresses and it looks like Leah is going to get kidnapped by the Demon King, Clovis. Letitia realizes that in order to stop the world from ending, she needs to step up, take her place, and somehow convince Clovis not to go to war. She thinks things will all go according to plan, until she discovers that Clovis was intent on doing all sorts of hot and naughty things with Leah in bed... which he's now doing to Letitia, of course. Hopefully, Letitia's sacrifice will pay off in the end and she'll finally be able to live the peaceful life she's always dreamed of!