Katekyo na Onee-san to Boku Ikenai Kojin Lesson

The protagonist is an ordinary student who doesn't stand out at school, with few friends and no girlfriend. His older sister, Mitsuki, on the other hand, is active, sociable, has many friends, and even has a boyfriend, leading a lively student life. The protagonist had given up, thinking that he'd never be like his sister and would probably just continue living a quiet life...
But something changed in his life. An encounter with a woman.

"Nice to meet you. My name is Moriuchi Honoka, and I'll be tutoring you"

"...!!"

Honoka was an older woman who had come to tutor the protagonist's sister.
With her generous heart and gentle smile, the protagonist fell in love at first sight.
Since then, he's spent his days thinking about Honoka, fantasizing about having sex with her, and comforting himself. After all, it's just a fantasy. It's never going to become reality...

But then, an unexpected event occurs. Honoka happened to witness him masturbating while using her as a sexual stimulant!
The protagonist was flustered and confused...
But instead of getting angry or scorning, Honoka smiled gently and asked,
"Is this how you wanted me to do it?"

"Huh?!"
Then she stroked him with her hand and made him cum.

As the protagonist was stunned by this unexpected turn of events, Honoka sweetly whispered,
"I'll teach you all sorts of things so you won't have any trouble when you get a girlfriend."

And so began Honoka's secret "private lessons."
The protagonist's fantasy became reality, and he was in a dreamlike state as he received her erotic instruction...

Then, one day, his sister, Mitsuki, came across their "lesson"!
"Wh-Wh-What are you doing!?"

Mitsuki was speechless, but Honoka persuaded her, and before she knew it, she was receiving the same "lessons" from Honoka as her brother.

Now that the protagonist has entered into forbidden relationships not only with Honoka, but also with his sister, what will become of his erotic love life?!

[From Getchu]