A marriage is void if one party to the marriage is related to the other as:
(1) an ancestor or descendant, by blood or adoption;
(2) a brother or sister, of the whole or half blood or by adoption;
(3) a parent's brother or sister, of the whole or half blood or by adoption; or
(4) a son or daughter of a brother or sister, of the whole or half blood or by adoption.
Official text of the Texas Family Code as published by the Texas Legislative Council.