I would need more information before I could give you an answer. How much equity is in the house? Is your Mother in Law looking to have you simply assume or replace her existing mortgage and then walk away? When your Mother in Law moves into senior housing can she pay her expenses for 5 years or is she going to be on Medicaid?
As a general rule, it is better to own a house with a mortgage, than rent to own, but it all depends of the prices, the interest rate and/or amount of equity you build up with each payment on the rent to own. If you have a rent to own, you want to have a lawyer familiar with such contracts review the contract before signing to advise you on whether you are adequately protected and whether or not the contract is in recordable form or that a memorandum of agreement is signed in recordable form. Even if recorded, you could still have the issue of any liens against the owner before you actually take title.
Whichever alternative you take, you will want to have a lawyer.
My answer to your question is for general purposes only and does not establish an attorney-client relationship,