Hello Shwana, Hysteria, also known as dissociative disorder is a condition which is often precipitated by stress. It can lead to various manifestations including episodes like seizures, motor or sensory loss, memory loss or as in your sister s case, regressing to past. I would suggest that you seek a psychiatric consultation for her. Medicines like escitalopram might be helpful for her, but the larger part of her treatment will involve non-pharmacological measures. It is a condition in which more the attention is given to patient s symptoms, higher is the chance of increase in symptoms. So, the first thing to do will be to reduce attention to her symptoms. It is important to understand what is she stressed about. It might be possible that the patient herself will deny being stressed, but it is important to be patient, give time and deal accordingly. She will also need to do some relaxation exercises. Your psychiatrist can guide you regarding all this. Hope that this information was helpful. Best wishes.