@Arstafas say that's an interesting turn of events, I like that idea....
Here's a crazy theory for you: Maybe Kohaku actually IS of royal blood, and he's just an illegitimate bastard that it would have been awkward to leave around. Hisui's mom or maybe dad, then used to work as a servant at the palace and either ran away with the infant child or the mother entrusted him to them. So when pretty much everyone else in the fmaily was dead and the last guy standing was half-dead, they remembered the bastard and took him back.