ツンデレついんず水泳部～オトナのレッスンしてください～

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]