Responses
There's no law against marrying a step sibling. It's not a blood relative relationship. See this. It's not incest, so there's no legal, moral or biological problems with marrying.
If I am not wrong, in reality same-sex marriage is prohibited in south Korea. So anyways they are against the law.
Real life situation: if so that their parents married and they become, as we see it, stepbrothers, is it illegal then for them to get involved romantically?