C.R.S. § 14-15-105

(1) An individual shall not enter into a civil union with an ancestor or a descendant or with a brother or a sister, whether the relationship is by the half or the whole blood. (2) An individual shall not enter into a civil union with an uncle or aunt or with a niece or nephew, whether the relationship is by the half or the whole blood. (3) A civil union between persons prohibited from entering into a civil union by subsection (1) or (2) of this section is void.