The protagonist is a Japanese history teacher at a private school.
One day, he gets a call from his uncle, who happens to be a loan shark.
Apparently, the parents of two famously beautiful sisters—students at the school—have gone bankrupt.
The uncle suggests sending the girls into the sex trade to pay off their massive debt. Hearing this, the protagonist hatches a plan.

“I’ll take on that debt for you. But in exchange…”

The thought of having those two sisters at his mercy makes him so excited he can barely contain himself.
And so, the older sister agrees to offer her body to protect her younger sister… while the younger sister agrees to do the same to save her older sister…

[Translated from dlsite]