Information about your sister's symptoms are not much helpful to me. You say she remains OK inbetween the attacks, she shows split personality Behaviour ?, does not talk normally, crys most of the time & realises that her mind is confused. Though not very clear but most probably it goes in favour of Affective type of mental problem because in Old Psychotic case [? Schizophrenia], Client usually lacks Insight which your sister still understand her problem. You need help of Psychiatrist
either by direct consultation or chating may be helpful to know her problems in details. I feel she should be cured.