It is possible to continue experiencing mild psychotic symptoms when taking antipsychotic medication but generally it is a sign that the dosage needs to be adjusted or that the patient is not taking his or her medication. The fact that one or both of his sisters continue to have incidents of violence and continue to experience significant psychotic symptoms in all likelihood means that there is a problem with medication.
Approximately 50 percent of individuals with schizophrenia do not believe they are ill and refuse to consistently take medication. It is immensely difficult, and in many cases impossible, to force individuals to take their medication. Even patients in heavily supervised hospital settings are able to avoid taking their medication.
Your mother-in-law’s expectations are unrealistic. She made a decision to care for her daughters in her home but you are free to make a different choice. Ultimately, the choice is yours. Their disorders are not well controlled. They require the treatment of trained mental health professionals who have the knowledge and experience to properly care for individuals with serious mental illnesses. It is unrealistic to believe that you could properly care for them. I would strongly advise against it. Please take care.