Brocon Oniisan, Kocchi Muite

After his angelic brother's birth 17 years ago, Haruhisa became an extreme brocon the moment he laid eyes on him. He poured all his love into his little brother in their busy parents' stead, putting his own needs aside. On the day of his precious brother's teacher-parent conference, Haruhisa marches in, ready to evaluate homeroom teacher Mitsuhara's worth. A few days later, Haruhisa discovers that his brother has a lover! Moreover, it's a male student from the same high school!? Turning to Mitsuhara to find out the truth, Haruhisa is suddenly kissed for some reason...?

(Source: Kaiohsha, translated)