Presumably the Form 1099-R for the $7,500 rollover has code G in box 7 and $7,500 in both boxes 1 and 2a. Enter this Form 1099-R in the same section as the one reporting the RMD distribution. Answer No to the question asking if the rollover was to a Roth 401(k), Roth 403(b) or Roth 457(b). Answer Yes to the question asking if the rollover was to a Roth IRA. TurboTax will include the $7,500 on line 5b with the rollover notation and will add it to 2023 taxable conversion basis on the IRA Information Worksheet.
(If the code-G Form 1099-R has a zero of blank box 2a, it means that the TSP was told that the rollover to to be to a traditional IRA, not to a Roth IRA, and your mother diverted the funds to a Roth IRA, which will require entering a substitute Form 1099-R (Form 4852), changing the amount in box 2a to $7,500 with explanation.)
