It should be legal for brothers & sisters (isn't it already?), it's a victimless crime and we shouldn't try to control who gets to have sex or how just because we're personally disgusted by it. Going by that logic gay sex would still be illegal.

For parent and son/daughter sex it's a little more tricky. Sometimes it's just harmless, kinky fun and sometimes the power/authority balance plays into it too much--especially when the son/daughter is under age, but that's already covered by "statutory rape" laws. I think the age of consent should be younger, but maybe not for parent/child incest. It sucks to tell fully consenting people they can't have sex, but maybe it's necessary to protect those who aren't actually fully consenting.

But given that the age of consent isn't going to change any time soon, do we really need separate laws to prevent parents and sons/daughters from having sex at ages 18 and over? I don't know, I'd have to hear more people's stories to know that.

I also think the laws for "statutory rape" and incest should be different for girls vs boys, because sexuality means very differe t things for girls vs boys and the social dynamics of it are different, but I know that won't happen any time soon because society isn't ready to face that fact and everyone would have conniptions.