Kanojo no Imouto to Kiss wo Shita.

Overview

After a month of dating, I was finally able to hold hands with Haruka, the first girlfriend I've ever had in my life. And on that day, my parents remarried and suddenly I had a step-sister who looked exactly like Haruka. Her name is Shigure, and it's no wonder they look alike. Shigure was Haruka's twin sister who became separated due to family reasons.

Source: AniList