Your girlfriend's sister is "better"

Being invited to play at the girlfriend's house, playing with the girlfriend is less, but playing with the older sister is more. Since you arrived here, the promiscuous older sister Ayami has always been attentive, even deliberately performing provocative and seductive actions towards you. Your younger brother is gentle and shy, but also lustful, so when he sees the older sister doing that, he can only occasionally sneak a few glances and quickly leave in embarrassment. Discovering this, the older sister approaches even closer, and not long after, the secret affairs between the two of them frequently occur behind the back of the innocent younger sister.

Surely the younger sister will regret it if she knows this, regret for bringing her boyfriend home, regret for trusting the promiscuous older sister too much..., but it's okay as everything between them is done secretly and furtively, making it very difficult to detect.