Tendou-ke Monogatari

Masato, a son of the Tendou family, is meant to marry Ran Houjou, the daughter of a baron. There's just one problem: she's a fake. The real Ran has fled after hearing that few make it out of the Tendou family alive. In her place is a young woman who says she will die if it means saving someone else's life.

(Source: One Peace Books)