Dealing with a sister who is mentally ill and lives with the couple

Question:
She says: I got married and I live with my wife and my sister, who suffers from insanity, in the same house. My wife wants a Shari'ah solution to this problem, and sometimes she asks me for a divorce. So why are you directing me?

Answer:
May Allah help you with that. This is a calamity, so you must be patient with it. You must restrain your sister from harming your wife. I seek refuge in Allah from lack of control. You must convince your wife to be patient with her, and you have no choice but this: patience, restraining your sister from harm and transgression, and convincing your wife and encouraging her by the reward and recompense for doing so. Yes.