Watakushi no Koto ga Daikirai na Gitei ga Goei Kishi ni Narimashita Jitsu wa Dekiai Sareteita tte Hontou Nano!?

Synopsis

Willemina, the daughter of a duke, suddenly acquires a new stepbrother, Nigel, and treats him harshly, believing him to be her father's "illegitimate child." Although she gradually comes to accept him as her stepbrother, she is unable to apologize for several years. Her stepbrother is set to become her guardian knight, but something seems to be going on. (Source: Kadokawa, translated)