The US does not have a blanket law but all states and DC have their own laws. Like in Ohio, it's only illegal to have sex with a parental figure, and in NJ, it's legal if both partners are consenting adults 18 or older, but if you're in Massachusetts and you have sex with any relative closer than a first cousin, you can get up to 20 years in prison.

It's illegal in NZ though and punishable with up to 10 years in prison.