Brother My Brother...

(from ebookjapan):

Nanako enrolls in Seiran Academy, a prestigious girls' school, which is a girl's paradise for gorgeous students known as " Lady Kaoru," "Saint Juste," and so on. The selection committee for the school's social group "Sorority," of which every new student dreams of joining, begins.

Sorority members are selected by upperclassmen, who vote on their family background, education, appearance, and other factors. Nanako thought she had no connection to this world, but for some reason she was chosen as a candidate...?

Contains 2 oneshots in volume 2: 'Summer in Canterbury', & in volume 3: 'Early Spring Trembling'.