Hakkou na Koushaku Reijou (Byoujaku) ni, Nokori no Jinsei wo Takusaremashite

Reincarnated in another world, Kureha, the sickly daughter of a duke, transforms from a militant office lady named Kureha! While bewildered by her new, delicate body and her brother Theobald's doting on her, Kureha sneaks out of her overprotective brother's sight and heads out onto the town. There, she sees a woman about to fall prey to some rough-looking men... Unable to overlook evil no matter where she is, Kureha confronts the men. She shares a secret with Isaac, the prince of a neighboring country, who happens to be there and witnesses the outcome of the battle...

(Source: Kadokawa, translated)