In the real world, I just find it gross, but in fiction, what people, write, people write, I have an incest story that's really dramatic and such, that I finished like a year ago, actually that I'm really proud of as well.
But they're not blood related, nor do they have the same last name, most likely. It would be the same with adopted siblings. They're not blood related, so they're perfectly fine, going ahead and falling in love with each other...or...whatever they want to do.
Eh, step-brother's and sister's....my mom would think it was weird and stuff, but it's not really though. Think, wasn't it that movie, Cruel Intentions that had a story line like that? I can't remember....there was a movie that had a step-brother falls in love with step-sister story line.
I wouldn't call it incest, at any rate. They're not blood related.
As for half-brother/sister, they're blood related, so yes?....I dunno....
Love is just love, no matter who it's with, nobody can help it.