> Why not allow 12 year olds to marry as a consititutional right?
12 year olds can't provide informed consent.
> Why not allow brothers and sisters to marry?
I actually don't object to this, so long as they're not permitted to have biological children. The primary reason why incest is bad is because of the genetic damage caused by it; eliminate that problem (by preventing the possibility of said damage), and there's not really that much of a reason left to forbid it (other than a potential "ick" factor, but that's mostly subjective).
12 year olds can't provide informed consent.
> Why not allow brothers and sisters to marry?
I actually don't object to this, so long as they're not permitted to have biological children. The primary reason why incest is bad is because of the genetic damage caused by it; eliminate that problem (by preventing the possibility of said damage), and there's not really that much of a reason left to forbid it (other than a potential "ick" factor, but that's mostly subjective).