At first, Wataru had only one sister. However, after making his promise many years ago, said sister started to split into the twelve sisters he has now.All of his sisters remember the promise he made at the time, even though some of them would have not even been born at that time, or otherwise too young to remember it. Also, I read one interesting theory that each of the twelve sisters represents one aspect of the personality of a generic "little sister": Wataru's original sister was a combination of those personalities, which were then split by the "magic" of Promised Island.
In the anime, Promise Island doesn't really existIt's actually Purgatory, Coma-Land, a Lotus-Eater Machine...whatever. At first, Wataru realizes this, hence his Catch Phrase. But by the end, he's so worn down by the Mind Screw that he no longer cares. (Plus he's grown to love all of the Sisters, not caring if they're illusory, demons, or whatever.) Whether this is a good thing or not is, of course, up to the viewer.
Akio is the same one as Revolutionary Girl UtenaName's the Same, and has similar motives and methods, after all. One wonders if this means Mami is a fragment of Anthy.
The Sister Princesses are Little Sisters.Thus Wataru is a Big Daddy being brainwashed into loving them and protecting them forever.
The Series takes place in an alternate universe where the Incest Taboo either does not exist or is more like a "Guidline"This one is obvious
Wataru's father is somewhat a distant relative of Charles Zi Britanniaalthough in wataru's father's case: he had only 12 or below wives. not 108. BUT ALL AROUND THE WORLD!! guy sure knows how to live...
Promise Island is, or will be, a Wizarding SchoolPromise Island is still under construction. We can see this in Wateru's flashback where there were few buildings on the Island, also with how the Amusement Park shows up out of nowhere near the end. It was made by magic. This explains why Wateru doesn't remember anything and why the sisters don't know each other; their memories were wiped to keep the school secret until it was opened. Wateru and his sisters are the prospective first students. The Tree Spirit will be one of the teachers. Chikage is the only one who has training at this point, and will help teach the others. A mysterious purpose. Then again it could be for fun.