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Just because he's getting called "nii-chan" doesn't mean necessarily mean they're blood related as the manga already stated, nor is it inherently bad to use/do so, asians often associate extremely close friends with familiality, (e.g. childhood friends.) Literally words like hyung, gege, nii-chan/san, kuya, etc... depending on your country/nationality that pertains to an older brother can be used on older close male friends. It really doesn't take THAT much to understand that, especially if you've been exposed to asian media for a long time, considering that you're here, in this website, reading asian media.