Tsundere Twins Suieibu ~Otona no Lesson Shite Kudasai~

The protagonist, Kirishima Hayato, is a boy who attends a prestigious academy. He lives in the adjacent dormitory and is the captain of the swimming club.
He spends his days studying, participating in club activities, and generally graduating like normal...or so he thought...
But those peaceful days are thrown into disarray by his younger sisters, who enrolled at the academy this spring!
His twin sisters, Sakura and Tsubame, followed their older brother to the same academy and, naturally, joined their brother's swimming club.
After being bossed around by the sisters throughout club activities, the protagonist finally returns to his room and opens the door to find the two sisters he'd been with just a moment ago there!?
When he asks them why, they explain, "We don't have enough room, and since we're siblings, it's okay," and they end up sharing a room.

One day, after having trouble cleaning up after club activities, the protagonist heads to the shower room.
Suddenly, he hears voices coming from the shower room, even though no one should be there.
Peering in, the protagonist finds the younger sisters engaging in lesbian acts in the private shower room.
In fact, the twins have been in a relationship like this for a long time, and they always comforted each other.
The protagonist's heart started beating fast when he heard his sister, Sakura, mutter, "I really do love you, so I wish you'd be honest with your big brother."
Though confused, his lower body responded obediently, and he tried to leave, thinking, "I shouldn't look at this anymore. Let's get out of here..." but accidentally made a noise and was discovered.
The twins were surprised, but they came to the strange conclusion that they might as well share their big brother, and approached him.
Thus, the protagonist was swept away by his twin sisters and ended up taking both their virginities right then and there.
"Now, you better take responsibility for this!"
A strange romantic relationship is formed between the brother and the twins, and so begins a life of lust.

[From Getchu]