That said I'd actually be happy if they get closer purely as siblings. The NBR trope is so overdone and real adoptive families are so much more than that.
It shouldn't matter that he's adopted? She's grown her whole life thinking of him as her brother?
You guys are reading too much ecchi manga, I'm telling you.
@Tamerlane Yeah, she's going to be really torn up. It's one thing to have hardcore incestuous feelings for your sibling, but learning that your incestous feelings are suddenly not technically incestous is going to give the poor girl a breakdown.
@goarachnidlaser Obviously, adopted siblings that never held feelings for each other aren't going to gain feelings just because they realize that they aren't blood related. The problem is that the little sister had it extremely bad for her brother even before learning that he isn't adopted, so learning about the adoption will likely shift how she deals with her repressed feelings one way or another.