Back 10-20 years ago, if I found a game like this, then I would definitely pick it up. Great job with the aesthetics of the game. Your art style got a good signature that gives me an impression of mature version of Yoshi's Island. The art really has a positive charm around it. In fact, it's the reason why I clicked on the game in the front page of Newgrounds.
The character moves quite slow, and it is a bit annoying to move so slow. This pushes me back away from the game. However, when the ghost sibling does some boomerang action, the game draws me back in. Good. I like that ghost sibling. (Yes, I like weird out-of-the-world things)
The music is a bit strange - it adds to that eerie feeling. If that is what you want, then fine.
Level Progression-wise though, I don't see much of "interesting things". You collect lots of items in the first screen, but then... so what? After all these item picking? Then, I came across only a few enemies or two in the next area. The boomerang action and the spinning body shine... for only a brief moment because there are so few enemies. Your characters already have attractive actions but you need to design more creative/interesting context in level design to allow your characters to shine better and brighter to hook the players better in the beginning.
Also, one question though: do you want this to be an action platformer or puzzle platformer?
But other than that, keep up the good work. Your art already pulls me in. ;)