You have not clarified which personality disorder your sister was diagnosed with. Regarding medication, I am not able to advise as I am a licensed psychologist and not a psychiatrist. You need to contact a psychiatrist to evaluate your sister and they can then decide whether a medication is needed or a combination of medicine and psychotherapy are recommended. Is your sister in psychotherapy? Who diagnosed her with a personality disorder: her primary care physician, a psychologist, or a psychiatrist? What is their recommendation? As you can see, many factors need to be considered before making a decision regarding medicating her.
Take care and remain safe.
Dr. Lata Sonpal