Eh, y'know, Incest with parents, siblings and so on in my mind is so gross. In that, I would never do it. Somebody you've grown up with all the time - In my mind it's not normal, but then I guess society's views have played a large part in that. Cousin's etc is still gross but I can understand it more so, you're not with each other pretty much all the time growing up, you don't share almost identical genes, etc. You mightn't be close to them at all!
Also you've got to think if these people ever have kids - I'm not sure whether it's proven or not about whether they're at a higher risk of biological problems. But they're going be so messed up, their grandad is also their dad, and they're being bullied at school?? It's terrible, and sick. o:
Saying that, I do feel sorry for the story's you hear about long lost siblings falling in love and then finding out they're brother and sister and the courts banning them from being together :(
Also, I do support the view that whatever they get up to in their bedroom's up to them, I just wouldn't want to be subjected to it
