The children of Noah's children would have been cousins. Cousins can legally marry in many places throughout the world. In fact, some states in the good old U.S. allow cousins to marry.
The fact is the DNA does not support a limited number of people like Noah's family producing the whole human race in just 5000 years.
We have too many alleles in particular loci to go back to only a few people.
And, no creationist can ever explain why apes and humans share the vitamin C gene defect if they are not related by common descent.