Everything is great....except the gameplay
Storyline interesting, voice acting very good, art good, music good. You have all it takes to make a great game but...
Gameplay : 0
The difficulty is to low at beginning and then change super fast. I didn't upgrade until cloud levels and I didn't lift my sister before the last level of volcano. However, I could hardly manage to beat the cloud levels even with all upgrade I was able to purchase.
The gameplay is also boring. I was expecting a platformer with puzzles where you would have to let your sister behind or something. Now, all I have done until the last level of lava is to hold down "A" and move left right to change target (and lifted my sister once). And after the volcano, enemies are popping everywhere, I don't have the time to charge the thunder power I have purchased, walls are destroyed in no time.
The grab option is also useless because :
1) When you grab your sister, you can't shoot, jump or cast walls
2) there are only 3 places where you can put her, left center and right. Baddies come from left and right so you can't put her there as a monster will popup on her and start hurting her and center is the default value. In addition, some monsters go pretty fast so the time to grab the sister, move somewhere where there is no monster and put her down, the monsters you were trying to flee from will have catch on you. not really useful.
3) You can't jump when you have your sister, which mean that if you are surrounded(and this is always the case) and you want to flee somewhere you will have to take hits. I usually let the sister get half the hits and take the other half myself.
So let me recapitulate the most liketly situation : there are monsters from left and right attacking your sister and you, you want to flee. You take the sister(while taking hits) moving somewhere through the monsters (while taking hits) to put her somewhere where monsters spawns(yeah, great deal) and when you're done, monsters you just flee have almost reached you. I guess I could have improved my luck by putting a cloud and then run, but when I'm desperate enough to grab the sister, I usually don't have any cloud left.
To resume, I have found the gameplay boring, repetitive and some mechanics of it useless. How would I suggest to improve that? Well, here are a couple of suggestions that might help.
1) Increase the area of battle and put a mini map. You would also have to add some objective or treasure(like a big rock with gold inside at the end of the level but you have to leave sister alone to reach it) to force the player to go there, it will had space and movement to the game.
2) Add levels of height, You have a great jump, why not using it to add some platforms?
3) Maybe a "I'm overrun" button. There are too much monster, you have to go there, use it only once per level so you have to keep it for the worst time.
4) I'm reading my comment again and I feel like I'm talking about the old mario bros minigame where you had to jump under the platforms to defeat crabs and turtles. Maybe grab some idea of that and put them in your game could improve it.
5) Make it possible to jump with your sister, or else it's pretty much useless.
6) Maybe some flying monster with angle of attack or weapon change.
7) The ha, ya take this is really, really repetitive. I don't really have an idea on how to improve it... maybe add a general sound to the attack and only voice acting when a monster is defeated?
8) Add tougher enemies and more damage to weapon. Because two hits on a monster for the whole game start to get repetitive with time.
So yeah... I might sound harsh like this but as I said, everything in the game is great, but the game is not fun to play. So I would recommend to rethink the gameplay and change it because you definitely have the skills to make a good one.
Hope it helped.