Can my sisters ex-husbands girlfriend take my niece to the doctor saying she is her mom and get her vacinated for HPV?

My sister and her husband divorced about two years ago. They are currently going through court to finalize a parenting plan. Recently my sister went on vacation leaving her 12 year old daughter with her biological dad. When she retrurned her daughter told her that the exhusbands girlfriend had taken her to the doctor and got her vacinated for HPV and that she had made her older sister an appointment too. When they went to the doctor the girlfriend stated that she was my nieces mom. My questions is: Can she legally do this? Is this exceptable?