One important question to the concept as it is done so far: For which age will the game be designed??

I think it's important to answer this question before we can decide about the story and the style in detail. Because an older audience needs a, well let's say more "advanced" story than it would be for a younger age, where gameplay would be most important aspect and story line less important.

Example: When I think about the brother/sister with special abilities plot as we have it so far, I ask myself: Why do brother/sister have special abilities, why are they divided, why are they in a dreamworld and so on. If our gamne will be for children, we won't have to deal with those questions that much. A child propably won't ask for the reasons and the story could focus on the action. But if not, our story should clear up aspects like these.