The IRS sent me a letter saying I can't sign for her unless I have some kind of legal document.
Typically, in order to appoint yourself as the personal representative for a deceased taxpayer for IRS purposes, you would submit IRS Form 56. You do not have to have been appointed by a court (see: https://www.irs.gov/taxtopics/tc356.html) You can find the form and instructions here: https://www.irs.gov/pub/irs-pdf/f56.pdf
In order to claim the refund, you will need to file Form 1310, which has special provisions for when you are the sole surviving heir and no personal representative is appointed: https://www.irs.gov/pub/irs-pdf/f1310.pdf
You do want to be sure that you also follow any rules in your mother's home state regarding probate of an estate to dispose of her assets and debts. You may want to contact a probate attorney about this if you have not already done so.
Please note this information is general in nature and based solely on the information you have provided. This does not constitute specific legal advice.
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