Class no Gal ga, Nazeka Ore no Gimai to Nakayoku Natta.

I, high school student Shinji Nagumo, have a problem. My step-sister, Tsumugi, who arrived for family reasons, is unfriendly. We're living together, but not getting any closer. Things changed one day when Tsumugi brought over a "net friend." It was my classmate and popular gal Yua Takarai?! As the three of us spend time together, Tsumugi has begun to open up. And suddenly, Yua is really close at both school and at home?! I found Yua hard to deal with, but I became the only one who saw her kind, lonely real self, and her domestic side. "I'm like Nagumo-kun's wife. Is that okay?" "That's okay! Shin-nii, let's live together, the three of us!" Well, that's not bad, but... Coming home together and sleeping over together. Tsumugi assumed Yua was my lover, and for some reason she seemed to be happy. Every time she comes over, our relationship gradually becomes real?! Gal and step-sister. A warm and sweet romantic comedy with two girls! (Source: Kadokawa, translated)