The Older Sister (Tsukiko) was set up as a stereotype, but slipped into the plotline as a "surprise". We expect her to be unchanging (1D), but she's actually complicated!
The younger sister (Tsukushi) is really the main character, along with the boy. That she is "unchanging" is another "plot twist"!!! (Acts 2D but is VERY 3D!)
I think they've managed to take "typical anime situations" and toss a new twist on them! Which is why I liked the manga!
Also: It has a cat... I'm easily amused... >^..^<