When you meant by "entire game," did you mean select a brother at the beginning and play him from here on out, or give the player the option to switch brothers any time during the game? I am working on a game with multiple Dreamers too; once I put a clear-cut system in place, I'll get back to you.
There are two problems that need to be solved here, though:
- Unless you want every Dreamer to share the same RPG Maker Items (usually Effects in most games but Perks in my game), the game needs to remember which Items are carried by which Dreamer, using either Switches (ON = have it, OFF = don't have it) or Variables (how many of a certain item).
- To have the player switch characters, you will need to remove the old Dreamer from the party, add the new Dreamer, then adjust the Item inventory to match what the new Dreamer is carrying.