The IRS says this: To deduct taxes or interest on Schedule A (Form 1040), Itemized Deductions, you generally must be legally obligated to pay the expense and must have paid the expense during the year.
https://www.irs.gov/faqs/itemized-deductions-standard-deduction/other-deduction-questions/other-dedu...
If your in-laws are still on the deed, they have the right to deduct the portion of the mortgage interest and property tax that they actually paid. But from your description, in your situation they're taking the deductions but the payment is actually coming from you and your husband. So technically you and your husband are entitled to the deductions, and they are not.
So this is a situation that you and your family have to work out with each other.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
