How can an aunt be convinced to stop smoking out of fear of Allah and not just out of respect for her brother, especially since she believes it is obligatory to conceal from him what displeases Allah?

Smoking is forbidden by Islamic law because it is vile and entails great harms and corruptions. Allah has permitted good things and forbidden vile things. Allah Almighty says: "They ask you, 'What has been made lawful for them?' Say, 'All good things have been made lawful for you.'" And a smoker's shame before people, while having no shame before Allah, is a sign of weak faith and personality, and it is a characteristic of hypocrites. Allah Almighty says: "They conceal themselves from people, but they do not conceal themselves from Allah, and He is with them." Nevertheless, your aunt's concealing her smoking from your father is better than her openly declaring it, if she insists on doing it.