Roshutsu Mesu

Shota was an average kid with not-too-good study habits.
Arisa, his sister, was an idol figure and an excellent student.
To help inspire her little brother, Arisa makes a promise to Shota:

"Match my grades," she teases, "and I'll do anything for you."

Now he's got the goal of having his way with her.
At first he photographs her naked, beautiful body, but from there it escalates.

[From DLsite English]