I feel sorry for Wataru... one younger sister is a terror (to me anyway). He's got 12 of them to worry about.
There's also a sequel series, Sister Princess Repure, and I really gotta warn you against it. Each episode is split into two sections, the main storyline and a fairytale told by one of the sisters, each at about 12 minutes long... As you can imagine, this doesn't leave much room for any actual story (It's practically non-existant, as far as any plot progression is concerned, though the finish is nice) but the fiarytales are quite beautiful, mostly based off of old European classics, and are pretty well the only thing that makes the second series worth anything.
go for it.