We rarely see Yuri mangas like those. Most of them have nearly no plot -they get together and have sex, that's how Yuri is.
But this one has an interesting plot with interesting (and good-looking) characters. The arts are also beautiful. The only thing that I find weird is... they take the 'sister' thing a bit too seriously. It's not like they had the same father AND mother. They are making it look like they ALWAYS had been together since they're born.
Well, I still never enjoyed a Yuri manga this much. It is awesome. I'm glad it became so popular. And it's still increasing in popularity with each new chapter!
(The waiting is a torture.)
(I prefer Yuzu over Mei. <3)