You know, with a title like this, I can't help but feel it's misleading and irrelevant if the sister isn't even going to be a focus of the game. Sure she's an integral character, but she could have been anybody, and the focus would still be on the other girls. It's the equivalent of calling Scott Pilgrim vs. The World "My Chinese Girlfriend". If she's not even a persuable option, whether or not the title is technically true, it is misleading. I feel like I waited for hours on a train only to find that I've arrived in the wrong city. Perhaps if I talked to the other passengers beforehand, I might have realised sooner that this train isn't going to Detroit, it's name is Detroit.
Sorry for the extended metaphor and if it seems like I'm kicking a dead horse here, but this is a big issue; if it wasn't, no one would complain about it.
I don't mean to hate on the game in any other way however, it is beautifully crafted with humor in all the right places. The writing and flow is much smoother than most games out there, so keep up the good work, and maybe I'll catch some of your later games if you make something that catches my eye. Good Luck Sumodeine!