Sengoku Blood - Bara no Keiyaku

When twins are born to the Oda household, Karin is tossed aside as an unwanted child. The one person who gave her strength through her childhood was her brother, Oda Nobunaga. Now both adults, Nobunaga sets about his plan for world domination in order to unify the world and create a place of happiness for both his subjects and his sister. However, tragedy strikes as Nobunaga is attacked in his palace and murdered. Beside his death bed, Karin vows to take on her brother's dream and act as his substitute, even if it requires making a deal with a demon to do so...
(Source: MU)