I don't understand the concept of half-sister or half-brother. For me if you have the same parent - you're sister and brother. I mean it's stupid to look at chromosomes.
Well, it's less chromosomes and more DNA. Under very certain conditions it is possible that you share less DNA with a half-sibling than with a cousin.
It's also probalby on where you grew up. Where I grew up, people made
(and still make) exact distinctions on relationship status, e.g. Brother/Sister, Half-Brother/Half-Sister, Step-Brother/Step-Sister and so on - here it's also not incest if you have sex with Step-XXX. You get looked at funny, because it's family but the law dosn't give a fuck.