I like the two player idea with this brother sister theme. Maybe one is alive and the other one is dead which could explain why both kids see different styles of the same world.

The style could be a mix of addams family and beetle juice. A cute goth girl and her brother living trying to solve complex time and gravity based puzzles.
A great example for time based puzzles is "day of the tentacle". I love this game and its style

Maybe we need some sketches (drawings not prototypes) to catch the possible spirit of the game and decide which way to go. [Edit]Oops, you posted you screenies while I was typing this

It is hard not put tons of new ideas on top of the gravity/time feature and we should think about levels, its single feature (feature introduction), and the upcoming levels with combined features from previous levels,...

what do we need?
- a description of what the game is
AND (this might be even more important)
- a description of what the game IS NOT