Estate Law Questions? Ask an Estate Lawyer.
Your in a tough position because you should have filed for a small estate. How you were able to handle her finances without doing such is a credit to you.
AARP isn't the type of business that will issue another check made payable to you.
If your bank won't cash such then you may yet have to file for a small estate - it will probably cost you about $150.00 to do such but then you will be able to cash the check.
Technically you don't have the authority (legal or otherwise) to endorse her check - in fact, to do such is basically a type of fraud and/or forgery.