Is it obligatory to inform the new fiancée about a previous engagement, the reasons for its termination, and some past sins, for fear of them being exposed and the former fiancée appearing?

Do not tell your fiancée about the sins you committed and repented from, for the Messenger of Allah, peace and blessings be upon him, said: "Whoever commits any of these abominable acts, let him conceal himself with the concealment of Allah." As for your previous engagement, you are not obligated to disclose it. However, if you see a benefit in doing so or fear a harm, then inform her without falling into anything unlawful.