A married person is not bound to support a spouse's children by a former marriage. However, if the person receives the spouse's children into the family and supports them, it is presumed that the person does so as a parent, and in that case, the children are not liable to the person for their support and the person is not liable to the children for their services.
Married Person Not Bound For Support Of Spouse's Children By Former Marriage
Mont. Code Ann. § 40-6-217, under Obligations of Parents.
Mont. Code Ann. § 40-6-217