Hi Annie2121, 

If you can define your mother in law's house as your "second home," you may be able to deduct the interest that you pay on its mortgage from your taxable income. You'll need to ensure that the deed to the house is in your name before attempting to make this deduction. Here is a helpful link that addresses interest paid on a mobile home as a second home: Is mortgage interest on an RV or mobile home tax deductible?