I don't know if this has been asked before, but how is it possible that Yui is studying a grade higher than her sister? Yui's birthday is November 27th, Ui's is Febuary 22nd; either those are in the same year and both sisters will be studying in the same grade(Ui is older than Yui), or, since Yui is older, Yui will be 2 grades ahead of Ui, both of which are non-canon. Yui being just 3 months ahead of Ui is clearly impossible, even if they were twins.
