All societies divide people into those with whom it is acceptable to have sex and those with whom it is not.
However, who is in each group varies from culture to culture.
For example (biological) first cousins are sometimes prohibited and sometimes preferred partners.
Next: Theories of the incest taboo
We don't talk about the details of kinship systems much in this class. However, many systems of kinship differentiate between parallel and cross cousins. Your parallel cousins are your mother's sister's kids or your father's brother's kids. Your cross cousins are your mother's brother's kids or your father's sister's kids. When cultures differentiate between parallel and cross cousins, typically one class is forbidden and would be considered incest, but marriage with the other class is permitted and in some cases considered ideal. This demonstrates the weak linkage between biological concerns and taboos against sex with close relatives.
Another point worth thinking about is that although there is unquestionably a link between inbreeding and negative biological outcomes, pre-modern people might have had a difficult time noticing it. We can study this because we have complex statistical techniques. However, these were only invented within in the past two centuries. Further, as we saw earlier, the deaths of infants and young people were so common that a link between inbreeding and early death might have been hard to detect.