Otokano ~Otouto no Kanojo ga Bunkei de Tsuyome!?~

It's a summer night at home. Six months into our relationship, I finally go all the way with my humanities-major girlfriend. After a night of passion, we fall asleep tangled together. The next morning, I wake to a strange, ticklish sensation down there. I blink away the sleep and see someone groping me with a dazed look. But it's not my girlfriend— It's my ever-strict, dead-serious, science-major... older sister!