@Mikau Them being step siblings would just be awkward at most. It doesn't actually close off any doors for their relationship aside from some social stigma attached to it that would be completely unreasonable because they knew each other first. If they were blood siblings, yeah they wouldn't be able to get married. Step or adopted siblings that aren't blood related are totally fair game (in the places I'm familiar with including Japan unless I'm mistaken, and obviously laws may vary in other places).
Fun fact: There's no reason his severe bro con little sister couldn't be the winner if they just never get married. Incest between consenting adults isn't against the law in Japan. Really puts the step sibling thing in perspective, doesn't it? Now, the social stigma there would be quite severe, but there's nothing stopping them if that's how they wanna roll. This is in know way me saying I want to see the sister come out on top either, just taking this subject to its extreme for an example.