If two people are on a vet account but only one signs any papers and payment agreement are both responsible for the bill

few months ago my ex sister in laws pet needed to be seen by a vet. I called the vet and set up the appointment for her. The vet put both of our names on the file because i called even tho it was her pet . I went with her to the appointment for moral support and I watched her animal for a few days till she got the new cage she got set up. She paid part of the bill a week after the visit. And signed a payment plan. However she still owes part of the bill she hasn’t paid. fast forward to a few weeks ago my pet needed to be seen and the vet refused to see my animal due to the outstanding balance. They are claiming that it’s my outstanding bill even tho it’s for an animal that’s not mine, I’ve never signed a single paper at the office( my ex sister in law signed everything as it was her animal). They claim it was my responsibility because my names on the account and I was at the appointment. Now i no longer speak to my ex sister in law and she changed all contact info