No, a brother and sister cannot marry in South Korea, as civil law strictly prohibits marriage between blood relatives within defined degrees of consanguinity. Under the Civil Act of South Korea, marriages between biological siblings, parents and children, and close collateral relatives are explicitly void by law to protect genetic health and adhere to traditional legal codes. Furthermore, historical legal prohibitions historically invalidated marriages between individuals sharing the exact same surname and ancestral clan origin, though that specific restriction has since been reformed.