Illinois Statutes Chapter 750. Families § 75/25. Prohibited civil unions

(1) a civil union entered into prior to both parties attaining 18 years of age;

(2) a civil union entered into prior to the dissolution of a marriage or civil union or substantially similar legal relationship of one of the parties;

(3) a civil union between an ancestor and a descendant or between siblings whether the relationship is by the half or the whole blood or by adoption;

(4) a civil union between an aunt or uncle and a niece or nephew, whether the relationship is by the half or the whole blood or by adoption; and

(5) a civil union between first cousins.