It's too bad there haven't been any deeper doujins with these characters. They have a really weird relationship setup that would be fun to play around with. Yuzuko and Yukari are flirty with each other and incredibly flirty with Yui, but they don't seem to be against sharing her and actively try to push Yui and Chiho closer together. Then you have Kei, who is super possessive of Chiho but ends up warming up to the three Yu's, and Fumi, who sees Kei as her 'plaything'. But nope, we get nothing but random sex, futanari, and rape dojins.
Oh well. This one was fairly cute and innocent, at least.
....I wouldn't be opposed to seeing the three gang up on Okaa-san-sensei, though.