Sweet story. Fairly appropriately lengthed. The sequence where B-ko decides why her life is empty is a little summaryish (in particular, with respect to Kei); but then, that's how an introspective B-ko would be after reflection.
Makes me think , at any rate,

A nice short story. I like the interesting perspective that A-ko and B-ko need to fight each other to be happy, but I remain to be convinced that C-ko could have made it into college! :) I look forward to seeing more from you.