Sorry to hear about the difficulties faced by your elder sister. Although 7-8 out of 10 people get better, there can be very few, who may not get better from depression for various reasons. It can be frustrating to the patient and the Carers. It would be helpful to seek thorough assessment by a psychiatrist to review the diagnosis, medications, psycho-social stressors, etc and plan accordingly. Also it would be helpful to have a family meeting. Best wishes.