How can my 97 year old mother-in-law get on Medicaid?

Generally, medicaid will not help much if someone is living at home or with relatives. Depending on your state, she might be able to get it to help with doctors and other medical expenses but not likely housing, food, clothing, etc. Medicaid is administered by the state department of health and welfare. You can go to their local office to find out whether your MIL qualifies for assistance. The fastest way to do this is to complete and application and turn it in with supporting documentation like bank statements showing direct deposit of social security and balances of checking and savings. They will also want 1099's or other forms from Social security that state her annual income and medicare expenses plus statements from pharmacies for her prescription expenses.
Best of luck.

Your mother-in-law might qualify if she is considered "low income" and if she meets certain eligibility requirements. Here's an article that might help:

https://www.agingcare.com/141681