Enter the interest at the 1099-INT screen, even though you didn't actually get a form 1099-INT.
In TurboTax (TT), enter at:
- Federal Taxes tab (Personal in Home & Business)
- Wages & Income
- “I’ll choose what I work on” Button
Scroll down to:
-Interest & Dividends
-Interest on a 1099-INT
You also need to report the principal payments as income. You have inherited an installment sale, not a mortgage. You need to review your mother's previous income tax returns to gather the information needed to continue reporting the installment sale "as if" she was still alive.
From IRS Publiction 537:
"Transfer due to death. The transfer of an installment obligation (other than to a buyer) as a result of the death of the seller is not a disposition. Any unreported gain from the installment obligation is not treated as gross income to the decedent. No income is reported on the decedent's return due to the transfer. Whoever receives the installment obligation as a result of the seller's death is taxed on the installment payments the same as the seller would have been had the seller lived to receive the payments."
