俺の姉がこんなに貧乳のわけがない！？（Ore no Ane ga Konna ni Hinnyuu no Wake ga Nai!?）

The protagonist is an ordinary college student—except when it comes to his older sister.
His sister, much older than him, is a beautiful and capable woman, admired as the “Madonna” of her school where she works as a teacher. But that’s just her public persona. At home, she’s a tyrant, treating her younger brother like a servant.

One day, the protagonist ends up taking care of his sister after she comes home drunk. That’s when he notices something spilling out of her chest—a mountain of padding. Turns out, his sister is secretly flat-chested. When she realizes he’s seen her secret, she punches him across the room.

After calming down, his sister confesses the truth she’s been hiding all this time: she’s been using padding to fake her figure, and no one’s ever noticed. Because of this, she’s never been able to undress in front of anyone or even date a man. She threatens to kill him if he tells anyone, but it’s clear her drunken state and frustration over her flat chest—and her virginity—have pushed her to open up.

“What can I do to make my chest bigger?” she asks.

Without thinking, the protagonist replies, “Maybe if you massage them?”
His sister admits she’s tried it herself, but it didn’t work. Then, without realizing the implications, he adds, “Maybe if a man does it?”

Big mistake.
His sister forces him to massage her chest. As he does, he starts feeling… strange. Meanwhile, his sister, experiencing this for the first time, begins to enjoy it. The tension builds, and she reaches her climax.

Afterward, she falls asleep, and the protagonist assumes she’ll forget everything by morning. But when she wakes up, she casually tells him, “You’re in charge of making them bigger from now on,” before heading off to school like nothing happened.

And so begins the dangerous relationship between brother and sister…

[Translated from dlsite]