What is the ruling on the prayer of the imam and the congregants if the imam remembers during the prayer that he is in a state of ritual impurity (hadath), so he completes his prayer then repeats it alone after performing ablution? Is it obligatory for the congregants to repeat their prayer?

You have committed a great sin by intentionally praying without being in a state of purity (wudu'), but re-performing the prayer with wudu' absolves one of this sin. You must increase your repentance (tawbah) and seeking forgiveness (istighfar), and not be ashamed before Allah. As for your brother's prayer, scholars have two opinions: some do not oblige him to repeat it because he was unaware of your state, while others do oblige it. The safer course of action is to inform him of what happened so that he can repeat his prayer.