Watakushi no Koto ga Daikirai na Gitei ga Goei Kishi ni Narimashita: Jitsu wa Dekiai sareteita tte Hontou nano!?

A duke's daughter, Willemina, was harsh with her foster brother, Nigel, whom she suddenly had, thinking he was her father's "illegitimate son." She gradually comes to accept her foster brother, but cannot apologize for it for several years. She is not able to apologize, and after a few years, her foster brother becomes a knight-at-arms, but things are not what they seem.

(Source: Kadokawa, translated)