Imouto ga Oyome ni Ikenaku Natta Hi ~○gakusei no Kikenbi Ma○ko de Kozukuri Rape Haruyasumi~

First 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 virgin boy watched them with covetous eyes.
Every day he yearned for Tomomi and Mako, but he couldn't act.

One day, Tomomi was gangr*ped by a group of guys.
10 men came inside her unprotected, newly christened hole over 40 times.
Knowing this made the boy even hornier. He fantasized vulgarly.

"I want to be inside one of those sisters... I don't care which one."

Summer vacation came.
Tomomi and her parents took a vacation, leaving Mako alone for one week.
Mako had sex with her boyfriend, and the boy secretly watched.
At last he couldn't contain himself.

"Now's the time. I'm going to get that little girl..."

[From DLSite English]