Hi Flynny80, a belated welcome to the forum.
You may have decided a course of action by now, or sought advice elsewhere. However, it may encourage more knowledgeable people than me to respond if you explain your circumstances a bit more, such as what kind of visa are you applying for? It's probably best not to put anything on the application that you are not certain about. If you are worried about this lack of information regarding your wife's mother, I can't see it hurting if you explain the situation somewhere on the application.
People are usually very helpful on this forum , so hopefully you'll get some more replies soon.