The situation you describe does not necessarily fit the typical definition of a hallucination. Hallucinations are false sensory perceptions that occur without any external stimulus. In your sister's case, she believes she hears someone insulting or talking about her, but you mention that you never hear anything when you look outside.
The experience your sister is having is more likely related to a psychological phenomenon known as "auditory hallucinations" or "paracusia." Auditory hallucinations involve hearing sounds or voices that are not present in the external environment. They can be caused by various factors, including mental health conditions such as schizophrenia, bipolar disorder, severe depression, or substance abuse.
However, it's essential to consider that other psychological factors can also play a role in such experiences. For example, if your sister is dealing with significant stress, anxiety, or feelings of low self-esteem, she may be interpreting neutral sounds as negative comments about herself. This could be a form of cognitive distortion or a manifestation of her emotional state.
It's crucial for your sister to talk to a mental health professional about her experiences and feelings. They can conduct a thorough evaluation to determine the underlying cause and provide appropriate support or treatment. Whether it's related to an actual auditory hallucination or a psychological reaction, seeking professional help can help your sister better understand and manage her experiences.