Imouto ga Gakkou ni Ikanaku Natta Hi ~○gakusei no Kikenbi Ma○ko de Kounai Rape Nama Koubi~

Second part of a series.

Beautiful and curvy, Tomomi was in fact a shy junior student.
Then there was Mako, her precocious and popular younger sister.

A lecherous stalker watched her with covetous eyes.
His lust only grew since the gangbang that ruined her virginity.

Now it was summer again and Mako was housesitting for her absent family.
She was menstruating. No chance of pregnancy...

The boy's assault was intense. She fled toward the only
safe place she knew: the school. It was closed for holiday, but surely
there would be people to protect her from this stalker?

[From DLSite English]