Mental Behaviour is not constant. Clear?

Hello, My sister is suffering mental illnesses from last 7 years more and as of now she is under medication. But sometimes her behaviour towards family members is very uncomfortable and she used to sticky on particular things so. Some times she is fine and behaves like as normal.If she think on something then she used to talk and talk then she will not take food and medicines as well as she come back as normal. We all are very scared position now as per her recent behaviour. Is it possible permanently curable and we even don't know what is the deases name as she suffers. Can she marry or if we arrange her marriage is that would clear? Please advise or any specific doctor treatment is required kindly help.