25 CFR § 11.603 - Invalid or prohibited marriages.
(a) The following marriages are prohibited:
(1) A marriage entered into prior to the dissolution of an earlier marriage of one of the parties;
(2) 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;
(3) A marriage between an aunt and a nephew or between an uncle and a niece, whether the relationship is by the half or the whole blood, except as to marriages permitted by established tribal custom;
(4) A marriage prohibited by custom and usage of the tribe.
(b) Children born of a prohibited marriage are legitimate.
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