Wait, so despite her extreme shyness, she goes above and beyond to dress up sexy for the guy she likes. He shows zero positive reaction, in fact, he rejects her effort entirely, and she's... happy about it? What?
i think its not like she dresses sexy, i think shes more comfortable around him, and to the approaches she makes, i think he just think about it as her social anxiety and "stumbleness" making her do that
i know its hard to say this, but they might not end up together, because being that much time inside and having no interactions can make you fall in love (not actually love) with the first person you interact
and if they dont end up together im... myself if you know what i mean. i would love to see an arc of her finding out what she truly feels towards onii-san and it is truly love