Seiten no Hekigan

Even after one month of transferring to a new school, Natsuki keeps her distance from her classmates. Her childhood friend Haruka always tries to push her to be more sociable, but Natsuki just pushes her away. One day, while Natsuki was skipping class and eating lunch on the school balcony, a slime-like substance drops right on top of her. The slime slowly forms itself into... a human girl? An odd relationship between Natsuki and her new alien(?) sister begins.

(Source: MangaHelpers)