@cspangie, we are sorry for your loss. To answer your question about the calculation you see in TurboTax for the Recovery Rebate Credit , it is correct.
Regarding eligible individuals who died in 2020, the IRS states that the Recovery Rebate Credit may be claimed on line 30 of their 2020 tax return. This means that though your mother isn't eligible to receive a direct payment, the money will benefit her estate in the form of a refundable credit.
What if the Recovery Rebate Credit is higher than stimulus amount(s)?
Stimulus payments were meant to be an advance of the credit. That means that if the total of those two payments is less than the Recovery Rebate Credit amount, you can get the difference back in the form of a refund. Moreover, if you received more than the amount of the credit, no repayment is required. You get to keep the money. So don't worry - TurboTax has calculated your credit correctly.
Here is an IRS link that discusses the Recovery Rebate Credit in further detail.
