I should qualify that a bit:

There are laws against incest in most US states. A good lawyer can point to Lawrence v. Texas ⚠️ ↗ and have each and every one of these laws declared unconstitutional as applied to consenting adults.

Now, as for it being right or wrong:

If there is not an absolute moral authority, then it's up to you to decide what is morally right or wrong.*

If there is an absolute moral authority, refer to him/her/it for the answer. But good luck in getting other people to believe you've found THE moral authority for the universe or that your understanding of his/her/it's moral dictates is the correct one.


*Note that it is not up to you, as an individual, to decide what is legal or illegal or what you will and will not go to jail for. In fact, if you really believe that doing the right thing means violating the law, you may be obligated to do the right thing then turn yourself in to the authorities in a public way, to force them to face the wrongness of the law you just broke.