Well the law, anywhere where incest is prohibited by law, doesn't say "having kids from incest is illegal." It says: "incest is illegal." Should it, according to you, be amended just to prohibit having kids from incest?
And also, kids. You say the government may interfere in this one because of "kids". Does the government always have a right to interfere in dictate who may have kids and who may not? Can people with genetic incompatibilities who aren't siblings have kids if there's a high probability of an unhealthy kids resulting from that? Can insane people have kids? Can this man have 89 kids from his 30+ wives?