What is the ruling on treating a mentally ill brother with harshness and strictness, sometimes causing him emotional pain, and am I sinful for that?

The general principle is to treat your brother with gentleness, especially if he is ill, for the Prophet, peace and blessings be upon him, said: "O Aisha, Allah is gentle and loves gentleness, and He gives for gentleness what He does not give for harshness, and what He does not give for anything else." However, if your brother's welfare necessitates the use of strictness to prevent him from harming himself, then strictness is permissible to the extent of the need. Indeed, Shaykh al-Islam Ibn Taymiyyah mentioned that an insane person may be struck if he begins to harm himself. This is akin to striking for the benefit of animals, or administering medicine to a patient. It is advisable to consult specialized doctors regarding how to treat your brother, and that any strictness should be proportionate to the need and your brother's welfare, not for venting anger, along with patience and seeking refuge in Allah from Satan.