@uin (little bitch ass dumbass coward blocked me out of embarrassment I assume.)
They've literally barely met ever in their life. They grew up apart and he knew nothing about her. It's not like they were even remotely close as children nor as young adults. They're basically strangers without blood relation. If you doubted how deep their love can be for happening so fast I'd understand. But getting your panties twisted over simply calling each other brother and sister is weird. They didn't even treat each other as siblings until recently.
While i agree with your last comment, even hoped that they remain siblings in this story i have to say that they never really see each other as siblings anyway. Sigurd distanced himself and see juliette and her father as strangers maybe and from the 'my father dotes on her and ignored me' sentence from him we can assume they're not raised together.
I must've blocked out this sibling incest cause I don't remember this in the novel