Nothing provided it's consenting (rules out child abuse) not coerced (rules out inequitable family power dynamics) and precautions are taken to prevent inbreeding. The last bit is ethically tricky. Incest has a huge ick factor but we do not criminalize people's relationships where children could be born with disability. For example two people with HIV or inheritable disease or disorder, even more severe medical consequences, aren't stopped from having a relationship. Neither are people punished for drinking, smoking, or not taking prenatal vitamins, all of which can lead to deformity.
Even so, sterilizing surgery is a thing. As are same sex relationships. Both of which neutralizes that particular issue. Though, practically, tracking that legally is difficult.
Some levels of incest generally slide below the radar first cousins in some places, second cousins in others. But direct blood relatives isn't.