I'm not a regular visual novel player but that one really interested me. It has a cool story with cool characters. Everything flows nice. It's a fun novel to read/play.
There are only two things I need to note:
1, the "bad ending" has a good flow but everything happens so fast compared to the good ending. I know it is supossed to be an ending where you become evil but I think we could have something there as a reminder that they are people. What happened to the relation of them after this end? And her sister? Nothing's explained on that evil ending.
2, I've noticed a bug when I was returning to some points with the loading method. Like the character images glitching (the character image did not fit with the situation) and then the game suddenly crashing. Note that I played on the web version so I don't know if it happens on the .exe version.
Anyway, I really liked it! Great job!