My mother is 89, she has significant memory issues. My sister had her cosign for an auto loan. My mom has no idea what she signed.

You, plus possibly other siblings on the short end of the stick may want to contact an estate planning attorney/ elder care attorney about this.
See what they say
I got a recommended attorney from a friend, and for short 15-30 min discussions about topics , the attorney did not charge me for the initial advice. This could vary depending on the attorney of course. He said he would start charging if we wanted to file something official in court.

Take her to her primary physician to give her a cognitive and memory exam so that there is a legitimate medical record that has a date. You don't have to tell her that's what the appointment is for, you can just tell her it's her free Medicare annual wellness exam. Then discreetly hand the staff or doc a pre-written not that says who you are and that you have concerns about your Mom's cognitive abilities, and ask for a cognitive/memory exam. I know they will do it, they did this for me with my MIL.

This is fraud and theft and reportable to authorities.

Start with APS and ask for guidance or attend an attorney and request guidance.
Charges should be pressed and her name removed.

Do a fraud alert and lock her credit with all the credit bureaus.

Odds are that the banks are saying no because of her age.