My Sister Controls Me (2015)

Atsuhiko, a marine biology student, spends most of his time at home in a converted storage room, continuing his research. His peaceful life is disrupted by his younger sister, Miu. Once a child who followed him around, Miu has grown into a woman. Atsuhiko tries to avoid her, but finds a way to cope by privately imagining inappropriate scenarios involving her. One morning, Miu is in the room with him, dressed lightly and brushing her teeth. It appears she has recently started dating someone.