Hi, from what you have written your sister may be suffering from Psychosis. She may be fearful of people or feel that people are against her. She may also be having delusions, ie. she may be hearing people talking to her when there is actually no one around. Hence, you notice that she talks or mumbles to herself. She will need evaluation by a Psychiatrist, however it is usually difficult to get such patients to see a psychiatrist willingly, because they usually do not have insight that they have a problem. This disorder requires treatment in the form of medication. Counseling or therapy is only later, once the thought process is better. I would suggest you and your parents maybe,  take an appointment with a Psychiatrist. Give details of her behaviour . In some cases the Psychiatrist may suggest liquid medication drops which can be given to the patient. After she is better on these she can then be persuaded to see the Psychiatrist. The Psychiatrist may also do a home visit or admit and treat the patient occasionally. This is a treatable condition, but please do not delay in getting treatment because your sister may never willing agree to seeing a Psychiatrist. I hope this helps. All the best!