The account for which the Form 1099-INT was issued should have been frozen as of the date of her death, so the Form 1099-INT should be reporting only income received prior to death and goes on her final individual tax return. Assuming that this was not a Transfer On Death account, the funds from this account should then have been moved to the estate account and any subsequent interest earned reported on a From 1099-INT issued to the estate. If these actions were not taken promptly after her death, the Form 1099-INT issued in her name might include interest paid to her account after her death, and in this case the proper way to handle it would be to report the entire amount on Schedule B of her individual tax return, calculate the subtotal of interest income, enter a line showing the amount that is actually income to the estate, and subtract that from the subtotal to determine the amount of interest income that is to be included in her AGI on her individual tax return. You must then issue a Form 1099-INT to the estate for the estate's portion as a nominee distribution from her to the estate and this Form 1099-INT will be reported on the estate's income tax return if one is required to be filed. See How to report at the end of page 5 of IRS Pub 559: https://www.irs.gov/pub/irs-pdf/p559.pdf
