In asian culture its natural to call someone older than you as onii-san or onee-san, blood or non blood related or even complete strangers. china, korea, philippines, japan, indonesia etc. even if she already knew he is not her brother, i think calling him that is just for honorifics
In asian culture its natural to call someone older than you as onii-san or onee-san, blood or non blood related or even complete strangers. china, korea, philippines, japan, indonesia etc. even if she already knew he is not her brother, i think calling him that is just for honorifics