Sei Chishiki no Nai Imouto ni, Ofuro Ecchi o Oshiete Mita Kekka...

As we have gone through a lot of sex, my younger sister
Kurumi recently comes to my room after our parents go to bed.

We are now in a relationship just like lovers. This time in the absence
of our parents having a trip, we have an all-day sex spree together!

[From DLsite English]