Migawari Seijo wa Moudoku Koutei to Saikou no Tsugai wo Mezashimasu!

Overview

A heroine summoned to another world as a bonus for being a "saint"—what awaited her was a forced marriage to a bird-man with deadly poison...!? Kanzaki Rui was summoned to another world along with her twin sister. While her sister, who has an aptitude for all kinds of magic, is treated as a "saint," Rui, who has no aptitude for magic and only possesses "poison resistance," is treated terribly. One day, Rui is forced to marry the emperor of a neighboring country overrun with beastmen, as a substitute for her sister. The emperor himself is descended from a demonic bird that harbors deadly poison, and is known as the "Emperor of Poisonous Venom," possessing deadly poison all over his body. A "political marriage" between a lonely king and a substitute saint. The destinies of these two, who should never have intersected, begin to move little by little. (Source: Echo, translated)

Source: AniList