The reason why incest is medically bad is because it makes genetic birth defects more likely. Most genetic defects are recessive. If two people who both have the gene have children, the childrenm may inherit the gene from both parents, and it will materialize. But if someone with the bad gene marries someone without the gene, then their children have one good gene and one bad gene, and -- assuming it's receissive -- they will not manifest the defect.
Bad genes are mutations, that is, DNA damaged by toxic chemicals, radiation, etc. Adam and Eve were presumably created with no bad genes. It took time for mutations to accumulate. So incest was not a medical problem for the first generations.
Incest also has social implications, the confusion of roles of "sister" and "wife" or "brother" and "husband". That would have been less of an issue before the Flood, when people lived longer. If Eve lived to be circa 900 years old like Adam, and if she was fertile for the same percentage of her life as a modern woman. That is, if a modern woman is fertile from mid teens to mid 40's, circa 30 years or 1/2 to 1/3 of her life, then if Eve lived to be 900, she might have been fertile for 300 years. She could have had children decades or centuries apart, who would not have been raised together.
And by the way, if Eve was fertile for several hundred years, she could have had dozens or even a hundred or more children in her life. And none of them ever called.