While doing laundry, the younger sister's arousal was heightened by the smell of semen emanating from her older sister's underwear. Unable to bear it, she showed the underwear to her sister's boyfriend and demanded to ejaculate on her sister's buttocks! Her sister discovered this, and they fought over the man's penis! Ultimately, they engaged in a threesome, alternating ejaculation on each other's buttocks while wiping the underwear!! (Nan Hinano, Ayase Kokoro FTHTD-208)