Maria-sama ga Miteru

Yumi Fukuzawa, who is basically a nobody and a first year student at Lillian Academy, a Catholic exclusive school for girls with a tradition that older girls take a younger girl as their little sister or "soeur," suddenly finds herself interacting with the members of the Yamayurikai and their soeurs when an innocent encounter leads Sachiko to impulsively ask Yumi to be her soeur.

(Source: ANN)