Sorry for your loss.
These are the IRS instructions. See if they answer your question.
Signature. If a personal representative has
been appointed, that person must sign the return.
If it is a joint return, the surviving spouse
also must sign it. If no personal representative
has been appointed, the surviving spouse (on a
joint return) signs the return and writes in the
signature area “Filing as surviving spouse.” If no
personal representative has been appointed
and if there is no surviving spouse, the person
in charge of the decedent's property must file
and sign the return as “personal representative.
https://www.irs.gov/pub/irs-pdf/p559.pdf page 4
You are correct you cannot sign with a power of attorney. The power of attorney is terminated upon the death of a the principal. After death financial transactions are handled by a successor trustee (in the case of a trust) or an executor (if there is a probate estate). In some cases an interested individual can do certain things. You should probably consult with an attorney (estate) in your mother's state to see what you need to do to about her estate/trust. (I'd start with the attorney who drafted her trust). But that is separate from filing her 2017 return.
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