It is illegal to marry your first cousin unless the woman is at least 55 years old.

The Law Explained

Wisconsin permits first cousins to marry only if the female cousin has reached the age of 55, or if either party submits medical proof of sterility. This highly specific law was enacted to prevent congenital anomalies in offspring while still allowing companionship marriages for older relatives. It remains one of the most oddly specific marriage regulations in the country.