What to say about the graphics and animations, they are all great, specially the walking cycle for the sisters, the poses while charging Sini's spells are gorgeous. And of course the CG's for the narration of the story.
I also enjoyed the variety of enemies and maps, even if is just a demo there is a lot to see and fight.
The boss battle is absolutely awesome, and it gives the best kind of Megaman/Metroid feels.
For little suggestions that I hope you don't mind:
-The game feels a little "zoomed in" and even tho that help on appreciating the art I feel like the maps are a little small and doesn't allow for a lot of movement. (Not that it ruins the experience or anything, it may be just me).
-My biggest problem was that dash on Shift key procs the fabled sticky keys and gets me killed a lot, specially on the boss.
-Sini's combo count is a little hard to get up, I didn't manage to see the SUPER and I really wanted to. I know is supposed to be the harder difficulty but maybe make the spells hit more times for the same damage.
-Maybe add a tutorial with the actual keys later, even though the controls are very intuitive, some people may have trouble following the tutorial without them.
-A map for later when the world gets huge.
-Make the boss text skippable after the first time maybe.
Again, I really loved this game, so much fun and really well done, this is just some friendly feedback, hope I didn't say too much.