No, in Chinese (and most asian countries) calling sister or 'Jiejie' (in Chinese) doesn't necessarily meaning they are biologically related, it's used as honorifics or term of endearment to address someone older (even by months) than the speaker.this might be an obsolete question since they're not human but: When the devil calls the angel 'sister' is that in the literal family way or in the colloquial-term-for-a-woman-you're-close-to kinda way~?
I'd be fine with either
No, in Chinese (and most asian countries) calling sister or 'Jiejie' (in Chinese) doesn't necessarily meaning they are biologically related, it's used as honorifics or term of endearment to address someone older (even by months) than the speaker.
But since they're celestial, I guess anything goes but I don't think the author is going for that