@MrsA so your sibling refuses to go to the doctor office with you? Then you should call the doctor's office to report their behavior because it is likely caused by dementia. Changes in mood or personality -- esp. combative behavior is a sign of dementia.

Memory Loss & 10 Early Signs of Alzheimer’s | alz.org

You really should have someone from the doctor's office come out to your home to assess your sibling, or you will be stuck as the caregiver without any resources for yourself.

You need to get your sibling medically assessed for dementia. Meanwhile, please call the Alzheimers Association and speak to a social worker who can email you resources. If you don't take any action then nothing will change and you will experience caregiver burnout, if you haven't already. I know it's hard but you need to take control.