Manga by author Yuuhi (夕日)

Wilhelmina, a noble daughter of a duke, suddenly finds herself with a new stepbrother one day.Believing him to be the result of her father's affair, she is stern with him, taking charge of teaching him manners and academics, which he clearly lacks.Despite her cold treatment, the boy, Nigel, never shows resentment.Instead, he calls her fondly: "Sister Sister"She thought he disliked her.But could it be that he's been deeply in love with her all along?A slow-burning romantic tale of a stoic stepbrother and a strong-willed noble girl, filled with hidden affection and growing love.