Koi Shita Hito wa, Imouto no Kawari ni Shindekure to Itta.

Wisteria, the daughter of a count, secretly has feelings for Bright, a beautiful childhood friend and son of a duke who, unable to use magic, had been conducting forbidden research in the shadows. Devastated when Bright confesses his love for her younger sister, Wisteria is even more hurt when he asks her to take her sister's place as a sacrifice. Is her love for Bright so strong that she will give up her life for him?

(Source: TOKYOPOP)