Mont. Code Ann. § 72-11-104

The degree of collateral kinship of two persons is computed by commencing with one of the persons, ascending from that person to a common ancestor, descending from that ancestor to the other person, and counting one degree for each person in the line of ascent and in the line of descent, exclusive of the person from whom the computation begins. Siblings are related to each other in the second degree, uncle and niece are related to each other in the third degree, and first cousins are related to each other in the fourth degree.