Cassmire: The Loyal Sword

Kashumir 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.

(Source: Zero Scans)