If You Lay a Hand on My Brother, You're All Dead

One day, she woke up from bed and opened her eyes to realise she has transmigrated into the BL novel Asterion's Starry Blue Night. The protagonist of a crazy story who killed himself on the verge of death. I've become that protagonist's older sister... Rosalite! The issue is that if and when my brother, Asterion, dies, the system will automatically revert me back to the sixteen-year-old Rosalite! That is why I must protect my brother from a plethora of other assholes in order to avert infinite regression! Whether it's from a son of wealthy household, a prince of the country, or even the imperial prince from across the sea, no matter whoever it is.