Imouto wa Kanojo ni Dekinai no ni

Yuki Sakuraba is the cutest little sister in the world. Although she is only in junior high, she is already as pretty as a model, has well-developed breasts, and has a great personality, too.
"Let's play a game! I'm going to beat up my brother today!"
"I picked out some clothes that I think you might like?
"Big brother, let's take a bath together. It's been a while."
But at home, she still acts like a child. I'm just enjoying the friendly sibling relationship with my cute little sister. That's what I thought....... until one day, when it was revealed that we're not actually blood-related siblings! It shouldn't be possible for my sister to be my girlfriend,...... but now things are different!
(Source: Dengeki Bunko, translated)