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This is true that people can suffer from this kind of illness. (most likely one is Schizophrenia
from description written by u)
They really hear a voices inaudible to others and have suspiciousness.
First important thing to help her is accept that this is kind of psychiatric illness which can be controllable but not curable (occurs due to chemical changes in brain),which requires attention of both doctors and caregivers.
Like Hypertension Diabetes
this is also kind of chronic illness which requires prolonged treatment and regular follow up.Usually patient doesn't have insight into illness so they don't feel to take any medicine for same (as they feel its true or voices tell her that by giving medicine u trying to harm her)
There are many options that psychiatrist
can help u by means of medicines,injections etc.
So pls take her to psychiatrist as earliest as possible,He will definitely help you to decrease the problem.