Koishita Hito wa, Imouto no Kawari ni Shindekure to Itta. Imouto to Kekkon shita Kataomoi Aite ga Naze Imasara Watashi no Moto ni? To Omottara

Lady Wisteria's greatest wish is to be worthy of the man she loves, Lord Bright. But when Wisteria's little sister, Rosalie, is selected to enter the Predawn Realm as its guardian, never to return, Bright begs Wisteria to take Rosalie's place! After all, Wisteria is skilled with magic, and resistant to the sinister miasma that threatens the mortal world. Heartbroken, Wisteria enters the Predawn Realm, accompanied by the sacred sword, Sartis. More than twenty years later, a stranger with a familiar face appears, demanding the sword and challenging Wisteria. Who is he, and why does he resemble Lord Bright...?

(Source: Tokyopop)