Kitsune no Yomeiri

Rin and Len are older sister and younger brother, siblings in a samurai family that rules in the land of Musashi. They lived there happily with their uncle, the current clan leader. Len has started to feel guilty about Rin's wooing. Then, one day, they fnd out that there have been talks with the land of Owari about Len taking part in a political marriage. Rin is against losing the man she loves, while Len accepts reality. The name of the girl who holds their fates in her hand is... Miku.

(Source: ComicWalker)