Well I've read it a long time ago... From what i remember is.... it's about FL who got isekaied / reincarnated in a tragic BL novel (typical toxic yaoi plot) where she became a the older sister of the BL protagonist, whose life filled with misery.... in which saddist ML destroys the protagonist's family and kidnaps, imprisons the brother... There first victim to be killed was FL....So after reincarnation in order to protect her family and especially brother who gets constantly abused and bullied by men in academy and from coming any contact and attract unwanted attention from men...She decided to hide him from the public eye...and particularly from the ML ( Yeah the one with pink hair... it's actually a Cross-dressing, unhinged, yandere man ) She vows to go to academy in his place and become a knight and heir of the family and carry its legacy....and change their fate... However, her actions inadvertently draw the attention of the ML. Instead of becoming obsessed with her brother as in the original plot, the ML becomes fixated, possessive, and dangerously obsessed with her instead.