Iowa Code § 595.19: Void marriages.

1. Marriages between the following persons who are related by blood are void:

a. Between a man and his father’s sister, mother’s sister, daughter, sister, son’s daughter, daughter’s daughter, brother’s daughter, or sister’s daughter.

b. Between a woman and her father’s brother, mother’s brother, son, brother, son’s son, daughter’s son, brother’s son, or sister’s son.

c. Between first cousins.

2. Marriages between persons either of whom has a husband or wife living are void, but, if the parties live and cohabit together after the death or divorce of the former husband or wife, such marriage shall be valid.