Kuro Gyaru Inmai Anna-chan ~Jimi na Imouto ga Henbou Shite Onii-chan Zenryoku Dokusen!~

As a tennis scholarship student and captain of the tennis club, our protagonist is dedicated to his studies and club activities at a boarding school and he is highly respected at school.
However, his true nature is that of a pervert who loves dark-skinned girls and collects images and videos of them on his computer and smartphone.
Though he hides his true nature, Anna can't help but notice.

One day, he learns that his younger sister will be attending the same school as the protagonist on a tennis recommendation, and he is genuinely happy.
Then, after the entrance ceremony, while the protagonist is out recruiting, a dark-skinned gal, the protagonist's type, visits the tennis club booth.

"Hey! Is this the tennis club? I'm here to join!"

Unable to contain his excitement, the protagonist panics as he tries to respond.

"Can you take me to the coach's place, brother?"

The dark-skinned girl turns out to be his younger sister, Anna, who has completely transformed from her plain appearance...

[From Getchu]