Sister Princess/WMG

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 exist
It'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.
 * This Troper can understand Mami deciding to stay - she's clearly 'gone native,' so to speak - but Akio? Does Wataru's rejection of the real/outside world convince Akio to stay as well, so he can start over with his friend and his sister? Or is the Akio who shows up in Wataru's class a convincing fake, conjured up by/for Wataru so he won't be tempted by guilt or friendship to leave?

Akio is the same one as in Revolutionary Girl Utena
Names the Same, and has similar motives and methods, after all. One wonders if this means Mami is a fragment of Anthy.
 * Akio's plot involves mistreating his sister (via neglect) and pursuing Wataru at everyone else's expense, it seems very likely that Wataru is some version of Utena-- An inverse reflection, perhaps, a timid/easily-led boy rather than a strong-willed, tomboyish girl. What shakes Akio so badly towards the end is Wataru finally displaying his Utena self, refusing to go along with others and 'be normal' by deciding to return to the island.

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 "Guideline"
This one is obvious.

Wataru's father is somewhat a distant relative of Charles Zi Britannia
although 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 School
Promise Island is still underconstruction. 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.

Promise Island is the board for a Cosmic Chess Game of Order vs. Chaos
Take a look at Wataru at the start of the series: he wears a grey suit, sunglasses, and his hair in the same style as everyone else. His future is laid out for him. All he has to do is follow the rules of behavior. Then his sister (agents of chaos) cause the errror in the test and the first thing they do is change his hair style and outfit to his own individual look. Everyday on Promise Island is unlike the next and his future can branch in different directions. Akio is an agent of order trying to pull him back. This is proven by the the first thing he does upon Wataru's return: change his hair and clothes back to the 'standard look'. Why are they doing this? A mysterious purpose. Then again it could be for fun.