Mo. Rev. Stat. § 451.020
All marriages between parents and children, including grandparents and grandchildren of every degree, between brothers and sisters of the half as well as the whole blood, between uncles and nieces, aunts and nephews, first cousins, and between persons who lack capacity to enter into a marriage contract, are presumptively void; and it shall be unlawful for any city, county or state official having authority to issue marriage licenses to issue such marriage licenses to the persons heretofore designated, and any such official who shall issue such licenses to the persons aforesaid knowing such persons to be within the prohibition of this section shall be deemed guilty of a misdemeanor; and this prohibition shall apply to persons born out of lawful wedlock as well as those in lawful wedlock. It shall be presumed that marriages between persons who lack capacity to enter into a marriage contract are prohibited unless the court having jurisdiction over such persons approves the marriage.
§ 451.020, RSMo
RSMo 1939 § 3361, A.L. 1961 p. 343, A.L. 1969 H.B. 564, A.L. 1983 S.B. 44 & 45