Hitomi no Rakuin ~Inbaku no Mesu Dorei~

Main character's sister finally returns home after long hospitalization, but at the same time parents go overseas leaving children to live alone. Sister asks to check on her classmate at night classroom, and protagonist witness classmate having sex there. Sister meets brother with red eyes and cries that she feels lonely. Peaceful school life changes dramatically since this day.

[From vndbreview]