Very nice game, really liked the custom piece rules and quite cute sprites and ran smoothly for me! As for suggestions, art-wise I think it would've been better if the other side uses pallete swaps of your pieces/characters, in particular since chess is supposed to be symmetric, also makes it easier to tell what the other side can do. Plus a bit more of story would've been nice, not asking anything too special but something like a few dialogue lines between the sisters between levels would've been cool, or maybe when you pick a new piece they say something to present themselves. Still enjoyed it quite a lot!
Also see some other people complaining about the AI, but considering the limited time and the use of custom chess rules and that the unit composition is always changing, I think you really did a great job with your AI. Many strategy games just make the AI blindly charge at you, but here sometimes the other pieces actually tried to maneuver around and aim at sniping the king sister, which is better than most others similar games I've tried.