Your younger sister, Riku-chan, is a bright, lively and obedient school girl. One day when her parents went out to stretch her wings, her sister brought her friends, and your silence disappeared in an instant. While being forced to teach her homework, she was forced to do naughty things, and when she got worried and went to check the bath, she was taken in by a group of naked girls... Finally, she was excessively provoked. As a result, I end up having sex with my sister without even wearing a condom.
