The limit for all resources for an individual is $2000, and that includes land, bank accounts, second cars, etc. In the divorce decree, it was decided that the two of them would share this property? A pretty dumb decision, in retrospect. What about rights to her former husband's pension? Did she sign that away also?
Your sister could buy out your mother's share so your mother gets the value of what she owns. Her ex-husband could buy her out also. You could buy her out. The value is what the land, as is, would sell for if it was listed on the open market. That could be a written estimate by an expert, a realtor who sells land with broken down houses in your area.
It is also odd to me that people keep land like this for sentimental value when they have no other way of supporting themselves.
Is your mother over 65 or disabled? When did she stop working? How has she supported herself since the divorce? What else does she own?