A mother-in-law in your dream implies the notion of management of unhappiness or stress brought on by years of living and dealing with family issues.
In a dream, a mother-in-law represents various things such as rigid opinions, blame and disappointment. Sort of like family members who are often disappointed in our choices and make snap judgments without giving us the benefit of the doubt. In the waking hours, find out where you are walking on the edge that would cause the concerns to arise from an outside 3rd party.