Seijo no Imouto no Shirinugui wo Ousetsukatta, Tada no Jijo de Gozaimasu

Dorothea, the daughter of a viscount, is compared to her selfish but beautiful younger sister, who has the title of "Saint," and has lived her life despised by both her family and those around her. One day, her younger sister insults the princess of an allied country, calling her "animal-like," which escalates into an international incident. Dorothea goes alone to apologize to the beast-man country of Rezanard on her sister's behalf, but...

(Source: Comic Earth Star, translated)