She is not a sister, she is not a girlfriend, she is a girlfriend sister.
It's kind of hilarious that most manga would be like, "We cannot utter our great sin." The main guy is like, "She is my girlfriend and my little sister; I don't give AF."
This series is becoming too confusing. Every chapter feels incredibly rushed. I think I'm going to drop this. If it finished in a few chapters, maybe finish reading. There are just too many plot lines.