Suki na Ko no Imouto

The one who fulfills my desires is the younger sister of the person I love―. A love triangle between the perfect older sister and her care-free younger sister. I, Yuka Amamiya, confessed my feelings to my first love, Nagisa Hanabusa... but I was rejected. Unable to let go of my lingering feelings, Nagisa's younger sister and my friend, Mino-chan, made a surprising offer. "I'll help you forget all about my sister." She offered to let me do everything I had wanted to do with Nagisa — but do it to her instead. I saw the shadow of the person I loved in her younger sister, and I started going on dates with Mino-chan, dressed as Nagisa, and... "Yuka-senpai, you're about to kiss a dear friend who's like a little sister, right?" I indulged in my desires, almost drowning in them. As I spent more time with Mino-chan, who was supposed to be a substitute for Nagisa, I found myself gradually falling for her― (Source: Kadokawa, translated)