I Tried To Be Her Loyal Sword

Kashimir grew up working as a mercenary to buy medicine for her precious younger half-sister, Aria.
On her way home, after completing a mission as usual

she trips and hits her head, causing her to remember her past life;

thus she realizes she had been reborn in the world of the novel <The Night of the Fairy>.
Her sister was the main character of this reverse-harem romance while she was one of the villains causing Aria to suffer hardships.
Since there is no way Kashumir would let her precious sister suffer

she vows to protect Aria until her adoption into the count’s family just like in the novel.
But things turn out differently, and even the male lead candidates

who should’ve been fish in Aria’s pond, all end up becoming obsessed

with Kashumir instead, when all she wanted was to protect her lovely sister.