Is it permissible in Islamic law to inherit from a mentally ill father who is classified as insane, or to take the benefits he used to give before his illness while leaving the principal amount?

Inheritance is not deserved except upon the death of the inheritee. The loss of legal capacity due to insanity or other reasons necessitates placing the person under interdiction (hajr), not taking their money. The jurists have unanimously agreed that insanity is one of the impediments to legal capacity, and it removes the capacity for performance if it is continuous. The matter of the interdicted person is referred to the judge to appoint a guardian over their money. This guardian disposes of their money as necessary and in the best interest of the interdicted person. Among their dispositions is spending on the interdicted person and on those whose maintenance is obligatory upon them, in a reasonable manner.