In Medieval England there was only two reasons that allowed a person to be divorced. One was adultery and the other was consanguinity. To divorce your wife all you need do is prove that she was related to you in the Third Degree and that she had received no dispensation. One penalty was that in the eyes of the church your children would be bastardized. The States also have consanguinity laws for protection of public morals.
Most states prohibit the marriage of first cousins.
Exceptions to this are New Jersey, New Mexico and New York. Woman are also prohibited in the same degree. Here in Massachusetts we have one of the most stringent consanguinity laws. If the reason for the consanguinity laws were to protect the public from a medical problem then why restrict a man from marrying his stepsister. They would not share a single gene.