Lala no Kekkon

Ramudan's twin sister Lala has been betrothed to a rich young merchant named Uruji since childhood, but when she falls in love with a different man, Ramudan offers to be her replacement during the wedding ceremony to buy his sister time to elope with her lover. But then he has to deal with the wedding night, and Uruji's unexpected reaction to his decidedly unfeminine physique!?

(Source: Libre, translated)