Imagination, Anger, Rude. Her treated.

My sister is suffering from strange illness which she got after returned from United States last year and we are unable to find the cause and reason behind her illness. She is talking imaginary things like some people are chasing her and her daughter and they are even hatch some conspiracy to kill her both. She has behaved very differently with mother, father and me ( brother ) and not expected such selfish, rude and angry talking from her. We have consulted very good doctor and get her medications and after 2 months of medications she turn to be normal and stable. After 2 months she has stopped taking her medications without telling the family and now the same problem persists again. Now the problem is that she is not ready to consult a doctor and even the medicines and any kind of treatment. So we want to know how we can pursue her to go to doctor and take medicines and if she will not be ready to go to doctor .... what other means or ways we can use to get her treated.