Montana Marriage Laws: What the State Constitution says…
(1) The following marriages are prohibited:
(a) a marriage entered into prior to the dissolution of an earlier marriage of one of the parties;
(b) a marriage between an ancestor and a descendant or between a brother and a sister, whether the relationship is by the half or the whole blood, or between first cousins;
(c) a marriage between an uncle and a niece or between an aunt and a nephew, whether the relationship is by the half or the whole blood;
(d) a marriage between persons of the same sex.
(2) Parties to a marriage prohibited under this section who cohabit after removal of the impediment are lawfully married as of the date of the removal of the impediment.
(3) Children born of a prohibited marriage are legitimate.
(4) A contractual relationship entered into for the purpose of achieving a civil relationship that is prohibited under subsection (1) is void as against public policy.
Montana Code Ann. 401-401
Source: Montana State Constitution