Last Boss Reisoku ni Korosareru Ane desu ga, Kare wo Suki ni Natte Shimaimashita

Fiona is reincarnated as a supporting character in an otome game where her stepbrother, Clovis, is tormented and brutally murdered. She intends to cherish Clovis to avoid his death, but before long, her feelings turn into love. Only the "heroine," the saint, can save him from becoming the Demon King... Fiona tries to hide her feelings, but is it too late for Clovis, who has received so much love?

(Source: Kadokawa, translated)