Mugetsu Ookamiza
well, technically in Japan relationships between cousins isn't really all that taboo. in some situations it's even encouraged (like in families with political or financial clout they may encourage relationships between cousins to keep the political/financial power secure with their family, considering how they kept a feudal culture longer than the Western world and how that kind of thing is perfectly normal in that kind of culture it's not that odd they'd have a holdover like that). so by Japanese standards there's really nothing inherently wrong with cousin relationships.
but really, you want a vaguely similar premise that's pulled off better go for Log Horizon.
I know it's normal, but if you were rose up like brother and sister, you just think that'd be weird, meaning that she's always had feels for him, meaning that she's liked him, even when she thought he was her real brother, right?