Kanojo no Ane wa Gal de Bitch de Yariman de

The protagonist is in a relationship with a deeply serious and reserved girlfriend, maintaining a respectful dynamic where they have not yet crossed the line into physical intimacy. However, his carefully controlled life is thrown into disarray by the presence of her older sister—a bold, heavily styled gyaru whose provocative demeanor and uninhibited nature stand in stark contrast to his partner's modest personality. The central conflict ignites when the older sister unexpectedly pulls him into her private room, completely upending his boundaries. Despite his inexperience and the fact that he has never been intimate with his actual girlfriend, the aggressively seductive older sister claims his virginity without hesitation. Driven by an insatiable appetite, she subjects him to a relentless series of encounters, repeatedly draining his stamina and leaving him entirely at her mercy. This sudden and overwhelming entanglement traps the protagonist in a compromising dilemma. Bound by a secret he can never reveal to his earnest girlfriend, he finds himself caught in the older sister's demanding grip, forced to navigate a situation that threatens to destroy his relationship and consume him entirely.