Catch-22 with Form 1310

Hi, I'm trying to figure out how to file my now-deceased sister's tax return for a small refund. What stymies me is that this is a 2021 return for a person who died in early 2022.

  • I am not a court-appointed personal representative, but I've been handling her financial affairs (including her tax returns) for several years. There are no other relatives and not much of an estate.
  • TT says to fill out a Form 1310, but I have already submitted a paper one. I don't want to confuse things by e-submitting another one.
  • If I don't complete the (unnecessary) Form 1310, the return has to be mailed. But a mailed return requires the taxpayer's signature ... which obviously can't be done.
  • The 1040 has a line for allowing a "third-party designee" who can discuss the return with IRS, but says nothing about a non-taxpayer signature.

Advice on how to break out of this spiral will be greatly appreciated.