Is the questioner sinful for not preventing his mother from applying makeup and perfume when she goes to work, with his father being aware of it?

If the mother goes out perfumed and adorned in front of non-mahram men, she is committing an evil and wrongful act, and some scholars have considered this among the major sins. The father is legally responsible for preventing his wife from doing so, otherwise he would be sinful. The mother should be advised gently, and you can seek help from some of her close relatives. If she does not respond, then leave her. Remind your father gently of his responsibility, and if he does not fulfill his duty, then leave him. By doing so, you would have fulfilled your obligation. Whoever reaches the age of responsibility is accountable before Allah the Almighty.