Marriage between second-degree relatives (aunt/uncle, niece/nephew) is already legal in Switzerland, but the new measure would overturn the ban on consensual sexual relationships between siblings, and between parents and their adult children. (Sexual relationships with underage children would, of course, remain illegal.)
As I reported Friday in response to news about a Columbia professor's arrest on incest charges, some U.S. courts prosecute incestuous adult relationships on the grounds that the government has a legitimate interest in preventing inbreeding. Other courts view children as forever-and-always minors when it comes to sexual relationships with their parents: Law professor J. Dean Carro, told me, "Regardless of the age of the child, there's still a theory that a parent is always a parent, a child is always a child and, as a result, there truly can't be a consensual sexual act."
The Switzerland measure rejects that thinking and allows for the possibility that an adult can meaningfully consent to sex with their parent or sibling. The Swiss would hardly be the first to allow for this: According to a 2007 report by the Max Planck Institute (via WRS), China, France, Israel, the Ivory Coast, the Netherlands, Russia, Spain and Turkey do not have any prohibitions on consensual incest between adults.