Not having interviewed your sister negates my ability to know what might be wrong. You mentioned that she was in a severe accident involving head trauma. It’s possible that she had sustained a traumatic brain injury (TBI). There are many examples in the scientific literature where an individual sustains head trauma and subsequently experiences a change in personality. A neurologist or a physician who specializes in TBI could provide insight into her symptoms. Medication and or psychotherapy could help.
Relatedly, you might try joining a support group for family members who have a loved one with TBI. It could be helpful to share your experiences with other people in similar circumstances. Try googling “TBI support groups” and your ZIP Code. There are support groups and organizations worldwide.
Ideally, your sister should be examined by a specialist to determine what might be wrong. An evaluation might uncover the source of her change in personality. Her doctor would be in the best position to treat her symptoms. I hope this helps you know how to proceed. Please take care.
Dr. Kristina Randle