There was no law change in 2020 that affects the calculation of the taxable amount of a distribution from an inherited Roth IRA. A distribution from an inherited Roth IRA will have code T or code Q, not code 4.
Both the online and desktop versions of TurboTax fail to ask you to provide the taxable amount. This is a bug in TurboTax. TurboTax used to ask, but somewhere along the line this functionality got broken.
The only workaround that I can think of is if there is no tax withholding shown on the Form 1099-R you can use code 4 instead of code T, mark the IRA/SEP/SIMPLE box, then as you proceed through the questions that follow, indicate that your mother made nondeductible traditional IRA contributions and enter the taxable amount that you yourself calculated for this distribution From the Roth IRA. Because there is no tax withholding, the details of the code that you entered on TurboTax's 1099-R form will not be forwarded to the IRS with your e-filing.
Note that if you mother made a contribution to any Roth IRA for some year prior to 2020, the distribution from the inherited Roth IRA is a nontaxable qualified distribution and you don't need to worry about this TurboTax bug.
