Just going to laser focus on this bit :)
But if you grow up seeing someone as your sibling and clearly have that kind of relationship, even to the point that you're still referring to each other as brother/sister during a marriage proposal, then yeah, you're siblings. You may not be getting any three armed, one-eyed babies out of the deal, but it's still a very jarring, unrealistic shift. Like, relatively speaking, that shit happens instantaneously.
But hey, maybe I'm wrong. Show of hands, who here wishes they weren't related to a sibling so they could bang them? C'mon, it's the internet, you're anonymous ^_~
In this case, Hoshido's attraction to Kamui is ironically perfectly natural, they've only just met as adults.
But hey, the game as a whole, I'm totally cool with. My most anticipated next to Zelda Wii U