@tickub: that might actually be her mum's old middle school uniform.
@gda19: On the contrary, I think that the way Mifumi writes them has improved over the past few volumes.
I'm finding it easier to put Nishikata's attitude down to being embarrassed at showing affection in front of Chi than to him genuinely having the mentality of a middle schooler.
His desire to 'win' notwithstanding, it's now clearer than ever that he loves and cares for his family deeply.