To many, it's very hard to see why. Sister Princess is not the world's greatest anime. The production quality is pretty low, as evidenced by the numerous animation mistakes and oddities that pop up nearly every episode. You'll definitely see them. There are times when an oddly drawn face will be on screen for upwards of five whole seconds without changing, so it's not really something you can ignore when watching. The artistic strengths of the show, however, lie in the colors and the world that they paint. It's a very bright show (especially on DVD) and the near-permanent sunny weather on Promised Island is exactly what this sort of show needs to create the atmosphere it wants. The backgrounds are rather sparse and you get the feeling that, outside of the named characters, no one else actually lives on the island, but it's all very effective in establishing the "feel" of the series.
The soundtrack also does a very good job of pulling the viewer in. It's perfect; so perfect that I actually listen to it in my spare time. Every piece of background music suits the show so perfectly that when I listen to it I am mentally transported straight back to the anime. The opening song, Love Destiny, is a FABULOUS piece of violin-heavy pop that I feel Yui Horie has yet to top. The ending is considerably less memorable, but Love Destiny has "instant classic" written all over it.
Though the story is rather thin, I feel the characters do a good job of holding it up. Each sister has a "gimmick", but you never really feel they're completely one-note personalities. It always seems like there's plenty more to them, even if the series doesn't explore much beyond their unique attributes. They're very entertaining, and when I watched it the first time I found myself growing to like and dislike the sisters as if I was the one living with them (even going so far as to periodically scream "UGH, SHUT UP, HINAKO", much to my family's confusion). Wataru is a relatively likable protagonist, and takes the sudden onset of having 12 unusual siblings in stride...which is, admittedly, surprising, but sudden popularity will likely do that to a guy of his type. The interaction between Wataru and the sisters is well done.
Overall, I still can't explain what I love so much about Sister Princess. It's paced slower than molasses, but I still often find myself longingly eying my box set, wishing for the "magic" that the show made me feel when I first watched it. It's greatest strength is pulling you into its world, and that's a world I've come to enjoy very much. It's safe, clean, and beautiful, and the worst things that ever happen are inclement weather and personal inadequacy. Sister Princess is, without a doubt, the anime I would most like to live in. I'm still not sure EXACTLY why, but a good deal of this feeling can be chalked up to the series' effectiveness in drawing me in. Give it a chance, it might grow on you. Maybe you'll get pulled in the same way I was.
Let's all meet on Promised Island!
