Is the questioner sinful for leaving his father with his mentally ill brother who assaults his mother and curses his father?

If your parents are harmed by living with your sick brother, and you can alleviate this harm from them—for example, by having them live with you—then you would be sinful for leaving them with him. However, if they are not harmed, then there is no sin upon you. It is incumbent to strive for the treatment of the sick brother if his verbal abuse towards his parents is due to a lack of awareness and choice.