Incest has consequences associated with inbreeding. Which is reason enough on its own.
Aren't we already inbreds already though? Also your wrong about the consequences of inbreeding. There might be a slight more chance of a baby developing an handicap, but no different from the average couple. All babies can be born with deformities, shall be ban heterosexual relationships too? You also make the mistake that two close relatives are engaging in sexual intercourse for reproduction. That might not be true. So, again, your argument falls short. From the atheistic point of view, you cannot say that incest is morally wrong. You just can't.