Montana Title 1. General Laws and Definitions § 1-1-219. Relationship by affinity

(1) Unless the context requires otherwise, in this code “affinity” means the relation that one spouse has, by virtue of the marriage, to blood relatives of the other. Therefore, a person has the same relation by affinity to that person's spouse's blood relatives as that person's spouse has to them by consanguinity and vice versa.

(2) Degrees of relationship by affinity are computed in the same manner as degrees of relationship by consanguinity.

(3) Notwithstanding subsection (1), the term “affinity” includes the relation of husband and wife. Husband and wife are considered to be related by affinity in the first degree.