What is the ruling of the Sharia regarding dealing with a mentally ill brother, and are the father and mother sinful for giving him medicine that might harm him, or for insisting on his fasting, or for not giving him what he desires, and is it permissible to place him in a mental institution?

We ask Allah to grant your brother healing. There is no sin upon the family if they consult a trustworthy doctor to increase his medication dosage and the doctor deems it beneficial. Fasting is obligatory upon him if he is of sound mind during the time of fasting. He is not commanded to fast if he is of unsound mind. It is not permissible to admit him to a mental hospital if it is highly probable that he will be harmed there.