I'm not an authority on ND property law, but often the basics are common across state lines: in the case of separate-owned property, often the non-owner spouse may join in signing of the OGL as a matter of identity to the mineral owner spouse ... signing of a lease/easement, etc. does not confer title. Also, though, at least in Texas: if a re-married couple occupy as their homestead what was one's separate property, then they've communitized the ownership of the property and it is no longer separatley-owned property of one spouse exclusive of the other. There should be explicit language in your mother's Last Will & Testament to address the succession of her separate-owned minerals. Likewise, she can make whatever disposition of the minerals as she may desire.