Was interest credited to the account throughout the year? If so, interest credited to the account between 1/1/23 and the date of death is taxable to your mother (even if she did not withdraw it, and even if you did not know about it). You should file an amended 2023 return to report the extra income on your mother's tax return.
Interest credited to the account after the date of her death is "income in respect of a decedent", assuming you are the person legally entitled to the money as her heir or representative. You can report it on your tax return as if it was paid directly to you. You do not need to issue any 1099 or 1096 forms. If you received the 2023 and 2024 interest in 2024, it is 2024 income. Report it as regular interest income.
If the amount that is taxable to you is less than reported on the form (because of the date issue I discussed), there are three ways to adjust it.
1. If you file by mail, attach a copy of the 1099-INT and a written explanation of why only part of the money is taxable to you.
2. If you are using Turbotax desktop version, switch to forms mode, go to schedule 1, and enter the adjustment on the line 24z worksheet with a one-sentence explanation ("interest reported as taxable by original account owner".)
3. If you are using turbotax online, go to the section on other miscellaneous income" and enter a negative adjustment with the same explanation. This will go on line 8z, which is not correct per the IRS but is an acceptable workaround when using turbotax online because you can't access line 24z.
Lastly, if the amount is small and you don't mind paying tax on all of it, you could probably just ignore the amending issue and report the entire amount as taxable to you in 2024. Depending on your mother's income and your income, you might pay more tax this way, but it would be easier, if not technically correct. You still don't issue any paperwork, it is still "income in respect of a decedent."
